I think they should go overboard and add characters from other Persona games, and if it turns out to be more successful than expected, some from the SMT universe.
My personal pick would be Rise but it appears she'll be just an annnouncer.
I just think it would be cool to have acharacter like Rise or Fuuka while being fragile, having a keep away style, instead of a rush down.
From P3 I'd be happy if they put Mitsuru and Akihiko.
Fromthe other games, they could add Maya from P2 IS and EP, it would be cool to promote another remake for the psp, the last one missing. Hell, they could add Joker as a hidden boss!