I wonder about that, too.
Does the AI, for example, choose spells based on the actual spell name, or based on what the spell does? If based on what the spell does, then it should use new attack spells just fine. Same for new items; if it chooses them based on what they do, then it will choose new items as easily as originals.
If we make a completely new effect, say a spell that behaves like a trap on the battlefield, I don't think the AI will know how to react to that at all. But maybe we can take that into account while designing new effects. Maybe we could assign some tag to this trap, for example, that makes the AI react to it as they would react to a unit that's much stronger than them--they'd run away from it. If it's possible to emulate those types of properties, a lot of modding possibilities open up. You could have a Fear spell, a Bait spell (makes the AI think a sovereign is at the target location, and he's much weaker than them), a Wall, maybe (if you can't make actual terrain appear, you could make a picture of terrain appear and make the AI think that terrain is impassible--same end result).
So a lot of mods might be made that play well with the AI, or even play off of the way AI works. But, some mods, I think, are going to ruin the AI, making the mods good for multiplayer only. (I bet at least a few updates will be made based on popular or interesting mods that don't work well with the AI. They'll implement some of those mods as features and make the AI work well with them.)