Soldiers or orcs could communicate and coordinate, for instance. So instead of simply walking the shortest path to the player to be dispatched one at a time, they could harry the player, use ranged weapons, retreat strategically, cut off the player's retreat, etc.

You could have something similar for tame monsters.

It would change the gameplay significantly, and require careful consideration of game balance. I think it would be more of a whole new variant, rather than something to tack onto an existing version of the game.

AI driven NPC interaction could also provide some amusement and make the game less predictable. You could give shopkeepers and other NPCs distinct personalities, where there behavior is driven by complex motivations.And I'm sure lots of other stuff.