The problem with AI, similarly to my stated problem with ability-locking, is that you're mostly depriving people of combat content with that.
I suppose people who pick easy difficulty aren't particularly desiring to experience combat content though, so I suppose it might be most effective to make their tactics very simple and straightforward. The problem I see is deciding what should be "normal" and what should be hard.
I think games like demons souls and dark souls had it more right in having their vision of the difficulty, where it is always hard but can be made harder not by some difficulty you toggled, but by your own restrictions you place upon yourself. A good idea would probably be to have an Ironman save mode.
I really do think that normal difficulty probably deserves and desires the full level of tactics enemies have to offer, and so locking any of it off seems like a bad idea (except those who want easy-mode, who I don't understand [why not read a book or something] but so be it, they can have their fun their way too).
I guess my ultimate point here is that difficulty will probably be more fulfilling and less content-restricting/limiting (be it AI, abilities, stats, etc.) if we have things like Ironman Mode instead, where the additional difficulties are at a meta-game level.
It's definitely something to be thought about, though. What good is having different difficulty levels in a turn-based RPG? There are more straightforward answers in shooters and real-time action games, but with turn-based, stat-based mechanics, difficulty setting seems to be very tricky and perhaps better left alone.
Keep in mind that also having more difficulties means more time balancing each one (lest you want a brokenly difficult mode).
