I'll just take a few ideas from the old "Hypothetical Movesets" thread, and add some new thoughts too.
Byakuren! I want to see her use her scroll actively as an extendable blunt weapon.
And have a super where the scroll runes fly from one end of the screen to another.
Now here's a thought. Byakren has a giant power backdrop like Yuyuko, and uses butterflies and all, so she could very much be nearly a Yuyuko clone.
And I'd like to see her use Shou and Nazrin like Yukari uses her two shikigami. That'd be funny. And a bad idea, atually.
Mokou. Who fights with her hands in her pockets, of course.
Flandre, who has a 5-card super in which she crushes the opponent's eye of tension.
I suppose Satori would also be a good character to implement, though I have absolute no idea how it would be done.
Characters who are not playable should have their own spellcard, acting like an Assist Trophy of sorts, for the lack of a better example.