I've said before, the two I'd want to see most in here would be SHIKI and Altrouge Brunestud.
And if they put Arihiko in:
1) It'd better be fucking funny
2) Nanako better be tagging along, and
3) If both of those aren't met I'm going to be pissed because nobody's going to ever seriously pick him, ever, meaning we wasted a slot that was more deserved by someone like SHIKI or Altrouge.
The only way the roster could really be expanded more is to dig down deep:
1) Shijyou Tsukasa. We got so many badass knife wielders; why not one who's horrible at it?
2) Yamase Maiko. AKA the girl Nero ate in the park. Sort of like a Mini-Nero Chaos, but with a few twists.
After that, though, we either have to dip down into normals, or else crossover territory.