Wolves would be a problem as it would put humans at a HUGE disadvantage in a game as their livestock woul be the only thing that is affected. However, how do the economies of each different faction work? If we could get that then possibly we could come up with some different creep ideas that could be thrown into maps to make it even.
I'd assume you could kill the wolves, i hate when units come from off-map. Perhaps to prevent randomness the wolves would come when the player reaches a certain number of livestock? It could also be a timer if preffered as well.