AI.
When you shoot a guy, in a large but dispersed group of enemies facing different directions from cover, and they all spin and immediately start shooting the exact spot you are hiding in, it pulls you a little out of the immersion.
Or scripted AI paths. It would be cool if it could reach a level where the AI is entirely random. Like a guard can stop for a smoke, or leave his post to take a piss in the toilet and stop to ask another guard to cover for him. They wouldn't walk in straight lines, or stand perfectly still for hours.
There's so much room for improvement with AI. However I don't think it's a high priority for most devs, probably because it's very difficult to program.








