Okay, so I've been a long time Zombie Fortress player and I recently started playing again and I realized why I stopped playing.
The game hates survivors
Now you may be wondering why I say that, I mean survivors get guns. Well this is immediately balanced out by blue having infinite respawns and red having an arena-like no respawn system, right? Wrong. When red dies they also become blue, which makes the game harder for any remaining red players. Okay, now you have to remember that there is a perk system which allows players to change what benefits they get at a moment's notice. Just one thing, they only apply when you respawn so guess who can change perks on demand? The 'zombies' of course. This means if the zombies are doing poorly because someone's selected a perk that gives them an advantage in a particular manner that you can just immediately on the fly change to a perk that perfectly counters it. Example? Take a Demoknight that has a full melee loadout, they're rather hard to approach because they can do a lot of damage, but if you use Tarred then they're slowed to a crawl and cannot properly defend themselves and can then just be kited to death. Engineers? No problem we removed the effort in this game with "Magnetic" a perk specially designed specifically for killing engineers. Pyros and explosives got you down? Try out this new combustible perk that requires little more than running in a straight line to get kills. Still not working? Just resort to good ol' Toxic, unavoidable damage that can be stacked with you and your friends to create a kill-field the size of a sports arena. And how do you get this magical killing field of unavoidable damage? By doing absolutely nothing at all! I mean who expects there to be effort required on the part of the 'zombie' team?!
Obviously this isn't balanced at all, so how can you fix it? Well the obvious answer is to reduce the "Hard Counter" perks that zombies have access too, these means Magnetic, Combustible, Toxic, Thieving, Marked, and Hunter. Why these perks? All of them counter a certain class or play-style of survivors. Hard. So what to do? These perks don't necessarily need to be removed, but should be altered to make them less advantageous against players who cannot possibly adapt to them.
Let's start off slow:
Zombie Perks
Marked: This perk lets you deal extra damage to a single person that is randomly selected. However, the problem comes in when there's only 2-4 survivors left so the actual requirement of who you gotta pick out of a crowd basically becomes nil. My suggested balance fix for this would scale the damage based off how many survivors are left. This means selecting it to kill the last 1-2 guys is no longer just free damage against survivors without penalty but instead becomes less effective as the game goes on.
Hunter: Hunter is a troubled little perk, it is basically Swarming+++. What do you get? Well you can appear anywhere (thus negating commute time), you have nearly instant respawn (just like swarming), and on top of that you get a not insubstantial damage bonus (70% in fact!) when you initially spawn. Right now this basically has rendered the actual perk 'Swarming' basically irrelevant.
My suggested fix for this would be to merge it in with Marked. How would this work? Well, strip out the quickened respawn time, bonus damage on spawn, and make the spawn marker a destructible prop (either by weapons or by touch). This works in well with the Marked's "Stalking of their prey" description which would make sense for them to spawn closer to their mark.
Toxic: Toxic right now is the perk you use when you've given up because the survivors are winning and want to kill them through damage over time while the 5 hunters on your team stand outside the door ready to pounce on them when they go to clear the toxic zombies. While 3 damage a second isn't a lot, this damage stacks with other toxic zombies, so if you have 6 toxic zombies you're dealing 15 damage a second to all survivors in a 600/HU radius. That's massive. The problem is also the fact that to get this advantage the toxic zombie need not do anything. In fact, if they do try to do something they lose this damage radius. So what should the solution be? I think it would be best if it were merged with Sick (for the damage-over-time poison), or changing the radius to be run on explosive logic (this being negated by walls and cover) but function at all times, even when the player is moving or attacking. Lastly, the damage-over-time mechanic needs to apply less poison for scout attacks since they get more of them.
Magnetic: Magnetic is one of those perks that I just don't know if you can fix it without completely altering it or just flat-out removing it. It's sole purpose is to kill engineers, they serve no other purpose. However, they also shut-down sentries for other zombies in a constant radius much like Toxic does. If I were to do anything besides just removing it or completely reworking it I would make the magnetic radius Explosive logic as well, requiring you to stick your head out for the benefit because you cannot hide behind walls to suppress sentries.
Thieving: Thieving is one of those perks that only complete assholes use. What does it do? Well it steals your ammo very quickly on hit (very fast for scouts). If all your ammo is taken they get a gun (somehow, despite them being 'zombies') and you're permanently out your primary weapon for the rest of the round. This is obviously not good. The way I would fix this weapon would be to make it sap ammo solely, no silly nonsensical gun for heavies or scouts, and no permanent crippling of a player. Now you might be saying "Well if they see a thieving zombies they should just switch to their secondary/melee" well there's a couple of problems with that. First off Thieving Zombies are indistinguishable from other zombies, they have no distinct skin color. Second, this is a team-based game, you know that whole teamfortress thing the game has going on? So often you will be with teammates who will naturally more effective to kill with primary weapons so people will often use this. That's a perfectly valid tactic. However the last bit is of course it's permanent removal of your primary weapon. With the exception of the Demoman and the Medic the primary weapons are pretty damn important, often if you lose your weapon you're pretty much screwed for the round. It's neither fair or fun, so I would like to see this aspect of thieving go out the door.
Lastly, they also drain 33% of a medic's Medigun charge, which is way too much; this number should probably be 5% for scouts, 8% for heavies, and 10% for spies. As spies have the slowest swing speed and least damage, they get the most benefit out of smacking someone; as scouts have the highest movement speed and fastest attacks they should steal the least. This should also apply for general ammo consumption. In addition, there is a bug which they automatically steal the cowmangler in one hit always when it is held out, despite it having "infinite" ammo and thus should be immune to 'theft'.
Combustible: Combustible is another perk like Toxic that basically punishes players for killing you. It requires no effort or activation on your part, you just run in; die; and rack up easy kills. The only way this doesn't happen currently is by hitting it with melee weapons. Now unless you are a demoknight you will probably be using your much more effective primary and secondary weapons (Plus who would use combustible against a demoknight???). This means you will cause them to explode, dealing a not minute amount of damage to you and any friends you have with you. Sure you die quicker, but you don't select this perk if there's a demoknight or sniper shooting you from far away. No you pick it when the survivors are holed up and using guns, explosives, and fire. So what would be my proposed fix? Well I would say that Combustible as is should probably be made into an active ability. In order to gain this "explosion" you should have to ready it somehow.
My idea would basically be once you press E you stop moving entirely (-1000 speed, -100% defense) and explode around you after 2-3 seconds if not killed. This gives survivors a chance to kill you before you explode, or vacate the area. This makes the perk require some actual thought, time, and effort to pull off instead of being the W+M1 scout's favorite perk. The base penalty would of course be an attack penalty because you're there for 'sploding not fighting.
As is I believe the perk is fundamentally flawed, unless fixed it basically removes the only advantage survivors have (guns) which isn't good for balance. This perk is obviously less effective in wide open maps like Volcano, but in those cases no one uses it.
Zombie Weapons
A lot of the zombie weapons are more or less straight upgrades when you consider that things like health penalties don't really matter when it's a slight amount and you get unlimited respawns and your opponents to not. So let's look at some of the weapons that are most often (ab)used by zombies. It is worth remembering that when it comes to melee weapons, most are just straight upgrades and this is well known in the tf2 meta.
Sandman: The sandman is an amazing weapon, it's a fairly good one and it's only penalty is that a point blank rocket may kill you instantly. That doesn't matter that much since the soldier has to take damage to do that and that you respawn and they do not. Second, it throws a stun-ball which can stun a player for ages in a video game. Since survivors are usually holed up in a small place, aiming them requires little effort. Second, your entire team can be using it, and can easily keep players stun locked to death which is not very fun for the survivors at all. Were it me, I'd probably change it away from a stun to some kind of other penalty (maybe marked for death like in MVM), and increase the sandman penalties somehow (either less damage or less health).
Cleaver: The cleaver combos perfectly with the sandman, which means that nearly any scout not using the Boston Basher will be using that weapon instead. What does it do? It inflicts bleed at distance, deals a fair amount of damage (even with it's existing penalty) but most importantly, if you hit someone stunned by the sandman it crits; which is a pretty much guaranteed kill if you can pull it off between the crit-damage and the bleed. And you probably can pull it off, you or your other 8 sandcleaver-scout teammates.
Boston Basher: The three-rune blade and the boston basher are effectively straight upgrades to the stock bat. Why? They inflict bleed with no penalty. Now you might be thinking "Well if they miss they hit themselves right?" Well kind of. They only deal 18 damage (half) to themselves plus bleed. But you must remember that scouts will not be swinging this till they get into melee, and death has no penalty for them which negates it's real disadvantages. They also have health regeneration which negates some of the bleed, and of course you only swing when near survivors anyway so it's not like you will often be killing yourself with the weapon. The advantages are of course obvious, +40 damage in the form of bleed. Absolutely painful.
Crit-a-Cola: This weapon is just plain problematic. It's received a buff recently that removes the minicrits you receive for using it. This means the paltry amount of extra damage you take (10%) is massively understated by the extra +35% damage you deal. If you happen to be using the boston basher, there's no reason to not use it since the cleaver doesn't provide any synergy with the basher. This reduces the amount of hits to normally kill with a weapon by one or two swings; add into it the bleed for 60 (or more damage, because minicrits) you're able to do way more damage than even the heavy can do. The normal swing deals 35+40 damage, so you have 75 after one hit. That's a lot for a single hit. Add in minicrits and you're dealing 47+50 which is nearly a one-hit kill on light classes, and a guaranteed two hit kill if they don't have a medic or health kit handy. This is all by yourself by the way. Oh yeah, did I mention this makes you move faster too???
Buffalo Steak Sandvich: This weapon has the same issue as the crit-a-cola, it was buffed recently negating it's only real downside in zombie fortress (being restricted to melee). In exchange you can deal an extra 35% damage with your weapon, letting you two-shot most classes; for taking slightly more damage from attacks (25%). Oh and what's even better is you get a speed boost while using it, negating the only real downside of another problematic weapon: The Fists of Steel. Remember that these damage bonuses players get on you still apply fall-off (unlike minicrits) so there's no real penalty for using it. If you die you die, oh well; you got another two dozen clones in the freezer able to repeat the tactic ad nauseum till it works. You can of course combine this with perks to deal extra damage with that perk, such as Combustible (minicrit exploisons!).
Fists of Steel: This weapon is another (like the combustible perk) that exists soley to negate the only advantage survivors have: guns. It forces you into melee with them, lest you waste your limited ammo (because most maps lack proper ammo kits). So what if you take double damage, you both have equal chance to hit one another (except for demoknights, because they have longer reach) so overall this is a win for you. When mixed with the buffalo-steak-sandvich the penalties of both are outweighed by the benefits, giving massive synergy.
Holiday Punch: This weapon is just the regular fists, but a nearly guaranteed kill if you strike them from behind. Even if you die the guy's stuck laughing for 4-5 seconds which leaves plenty of time for them to get killed. They really should have some kind of penalty associated with their use, because as is there is no reason to use them over the fists.
Bonk: So this weapon allows zombies to become temporarily invulnerable at the cost of being able to attack. This is not only bad mechanically but exploitable with the nature of some perks (such as Magnetic) where it allows you to get a passive benefit while invulnerable. Not to mention that this item lets you close in on survivors with impunity, negating their only advantage (ranged combat).
Zombie Mechanics
Backstabs: Backstabs have always been a balancing problem in zombie-fortress. Most of the perks simply allow you to backstab easier and since it always does x6 their health in damage it always killed unless you had a 85% damage penalty or more. In the Zombie Hell server this is slightly less of an issue, but still a fairly big issue as backstabs now just leave you at pitifully low health for ages. This is mainly a three-fold problem. First backstab hit detection has always been dodgy, when you mix in things like innate speed boosts and the like they happen more often. Second, the penalty does not scale down if you have a damage penalty. Backstabbing your mark has the same effect as backstabbing when you're Swarming. Third, the penalty is higher on classes with more hit points as it takes longer to refill and they're out their tanking ability.
My solution would be that whenever a player is backstabbed, their maximum health decreases by 50 but deals no damage otherwise. Engineers and Snipers backstabbed would have 75, then 25, then 0 (dead). Medics go from 150, to 100, to 50, to dead. Pyros and Demomen go from 175 to 125, to 75, to 25, to dead. Soldiers go from 200, to 150, to 100, to 50, to dead. So each stab basically reduces the amount of hits needed to kill them by other zombies by 1 for the duration, more or less. The characters that are struck with this penalty regain hit points at a steady rate, instead of all at once. They'd regain 10 maximum HP every 5 seconds until they reach their normal maximum again. However, it wouldn't be all doom and gloom for spies, each time they make a successful backstab they regain all their hit points and clear any conditions they have on them (afterburn, bleed, jarate, etc) and have an overheal like the Kunai, letting them possibly backstab again.
Showdown Taunt: This is another problematic issue with the game. Mainly that it's effectively a large unstoppable ranged weapon that the zombies can used if they stand still for a second or two. It stuns for 8 seconds and deals 100 damage. That's massive. Normally it's just an instant kill, but right now it's ever so slightly more merciful. However, personally speaking here I just don't like what it does or what it entails. First off it means the zombies stop acting like zombies and start playing cowboys and Indians (why?), which does not really make sense for the game mode. Second it's just plain too effective for what it does. Most survivor games involve survivors huddling in a small cramped room while zombies swarm them from the outside. That's fine, and that's how it's supposed to be. But instead of actually having to break down their barricades players often resort to the tactic of spamming the taunt in hopes of getting cheap, nearly effortless kills by shooting at fish in a barrel. It just doesn't seem to fit well with the experience the game is supposed to be producing.
Health and Ammo Kits; Currently as-is zombies can pick up health and ammo kits, which doesn't really make sense since they already have a base-line regeneration and of course the fact they're supposed to be zombies (thus what is life-preserving medicine going to do to help them?). I think it'd be better if zombies couldn't pick up those resources, many maps already put too few as is, and zombies should be zombies and not running back to get health kits like they have self-preservation or something.