I think that on the internet in general and in the gaming industry of late the "special" zombie lore has already increased by leaps and bounds... the real problem here is that TFP have
too much material to use for inspiration. So the most difficult part IMO will be choosing something that is in line with the general look and feel of the game.
- Up to now we know that the virus infects not only people who look "almost alive", but also long dead bodies.
- We also know it can affect dogs (?dire wolves?), bears and vultures, but for some reason does NOT affect chickens, rabbits, stags/does, boars, mountain lions and the normal wolves (oh, I forgot snakes!).
- We have seen mutations that give special abilities to the undead, like the jumping skill of the spider, the rallying cry of the screamer or the puking attack of the cop.
Based on these points TFP have a wide (but not infinite) range of possibilities for choosing the new special infected (if they wish to keep the current look & feel).
IMO they should also consider to stay in theme with the Arizona lore...