Some time ago there was discussion with regards to difficulty level. It got me thinking that perhaps the next 'frontier' isn't so much the immersion factor of games, the inventiveness of controls, or the graphical overhaul within the system. I feel that graphically we've reached a point where everything is already really impressive, engine and aesthetics. I feel the next frontier should be the AI.
At present, I feel that L4D and Halo:Reach have come extremely close with regards to this. The limitations though are that the L4D companions can't seem to throw projectiles, and seem to be much too happy healing you, and for Halo:Reach, the AI companions not being able to drive properly, but nonetheless are the creme of the crop with regards to AI; in terms of companion AI anyway. Then we have the complete opposite, seen in RE5, or most AI used for escort missions.
I feel that for the most part, enemy AI is sorted, though some still fall into the trap of following scripts in response, or being made more tough through damage and resistance, rather than truly learning player behavior. I think it would be fascinating to have an AI that adjusts accordingly to your play style. Sort of like how Battle.net organizes the appropriate league you belong in for online Starcraft 2.
Especially for companion AI, I think an improvement on this will absolutely put the game on a level above most others, and with the evolving enemy AI, would make your games feel more dynamic.