There have been some attempts to apply ML to NPC control: sometimes to learn how to beat the player, sometimes how to mimic them (Forza), sometimes to learn what player wants done and assist them (B&W). But none of them have crossed the chasm from novelty to killer app.
The main difficulty with combat AI is that players enjoy mastering a game and beating it. And if the game keeps getting better and beating them instead, they will end up unhappy.
Yeah I think that's a very interesting aspect. If you want, I think this speaks to a more general point about AI that I have thought about sometimes. I suspect that we enjoy interacting with machines and using machines when we understand them, and that this understanding of how a machine operates, its capacity and its behaviour, is similar to the "theory of mind" idea that we employ a model of another person when interacting with them. That is why it is so frustrating to work with a machine that changes as you use it -- it's like trying to shoot a moving target.
I really suspect that ML will find its place in helping design machines that are then static, but ML that adapts as we use it will be frustrating to work with. At least until we develop, as a society, some kind of "theory of mind" of AI, where we learn how to expect a machine to adapt as we use it. But we are not there yet.
For now, machines are still better when they act like machines. That is why I think, for example, society adapting to the presence of things like ubiquity of self-driving cars is going to be a long and scary road.
As for video games, it seems like a computer-controlled character should be somewhat predictable, even if complicated. When the ML algorithms adapt, it should present a new character to fight, not adapt an existing one that the player has already "theorized" (i.e. figured out its pattern.)
The main difficulty with combat AI is that players enjoy mastering a game and beating it. And if the game keeps getting better and beating them instead, they will end up unhappy.