The video includes much more and precise details than what I'm going to type out, so I suggest you watch that first. Below is "more important" and larger rumored things to come with this update or future updates.
At 2:23, "TF_Matchmaking_Tobor = 2;" could be a coincidence name for a new gamemode but I'll just leave this alternative unused sentry gun texture here... https://wiki.teamfortress.com/wiki/Tobor^this is actually matchmaking for mvm. i don't play mvm or know the matchmaking scripts so I don't know shit. disregard that completely.
According to testinglol, the person that is "predicted" this Spy and Engineer update as well as parts of Meet the Pyro and Love and War update, this rumored update is possibly going to introduce a 10th class, the Civilian. The Civilian is a class in TFC, for those of you that do not know, he was to act as the Payload Cart, VIP equivalent, in TFC. You were to escort this Civilian to a specific point of the map to win.
This update includes some scripts for supposed competitive matchmaking system in the base game for Highlander, 6s and alike as talked about at 1:30.
shit is going to happen folks.