Something I would find interesting to see develop in this game is how the various creatures and monstrosities of the apocalypse interact with one another.
This is likely a hefty consideration AI-wise but aside from the occasional Hulk plowing through its smaller cousins on the way to turn me into a meaty jam (or spitters and their teamkilling ways) I rarely see the creatures interacting in any significant way (or particularly the opportunity for players to exploit those interactions.) Predator-prey interactions could be a bit more lively, as well as intra-species interactions (for instance, the canines would almost always hunt and stalk in packs.)
How they interact with the zombies also puzzles me to an extent. Bears, wolves, giant spiders, moose, and about everything under the sun wants to hurt me… but essentially ignore the far easier to kill zombies. I understand the meat is tainted and unattractive to many predators, but surely the zombies would initiate hostilities (if only to spread the infection.)
Since the zombies only form the front-wave of the apocalypse I see improvements to species interactions having positive repercussions further down the road in the mid and late game as the environment starts to consist of stranger creatures than mere diseased corpses.