Ironically, Xion would be my top pick for a spin-off gameI liked TAV so I wouldn't mind a spinoff with them. Bbs is the only game that got the trio dynamic right to me.
I wouldn't mind a Roxas or Xion spinoff but Days did them both a disservice. So I'd be more skeptical going in. Following @Sonofjafar 's suggestion, choice based gameplay would be good for them.