AI when it comes to animals I'd like to see a breathing world without the need of the player to discover it. Animals hunting, plants growing while animals eat them, etc.
Simulations are not games, so integrating a simulation in a game is always a nice idea, but it brings a very heavy burden with it. You can compare it to hollywood movies, you don't need to simulate an whole ancient city with a million population to make 3 scene shoots in it.
The problem is, that the player would not recognise it as simulated world, there's even the danger of missinterpreting emerging behaviour as bug. L4D is a great example of a virtual populated world which is only simulated close to the players.