I'm surprised she hasn't been mentioned, at all, but I'd love to see Valvalis/Barbariccia. I don't really dig the "assist" much, and I think all characters should be playable. But, due to the fact that the assist is there, and for a valid reason..! I guess Valvalis would be more of an assist, though I'd prefer to see her playable.
And yes, I realize, she's already a summon. But I don't like that. XD It should be changed.