Like alot of the ideas here that I've read! Most of this is sorcerer related since, well, thats what I play.

Demonforms were mentioned I like this idea, would potentially make for interesting situations, maybe you could possibly get exiled from towns and each demon type would have their own abilities? This reminds me alot of Diablo

I also liked the idea of learning spells from Arcane circle (or maybe other circles altogether? Multiclassing was brought up a bit.), there's been times I felt I wanted to be a wizard or cleric, making some kind "blend" would be interesting.

Also something like unlockable magic stances might be interesting, I know FFXIV essentially does this. With a "MP Recovery" stance and a "Damage dealing" stance, to encourage you to use diffrent spells.