Even though it may be a little late to suggest a new idea, but instead of sticking to 6 defined sets, have the server scramble all the hales and randomly choose 23 hales from any set to be playable on that map.
Also, all us donators wish we could choose a hale from ANY set at any time. I do see one problem with that though - new users who haven't downloaded all the bosses yet. But one can wish...