I just reached the point in Persona 5 Strikers where Zenkichi gets his Persona (does it count as a spoiler if it's in the game's advertising?), and he's fun to play as. While I don't think we'll be getting a Persona 5 Arena game (I'm guessing that Strikers is meant as P5's equivalent to Arena, since it's a continuation of the story with a different developer and genre), I'd love to see Arc System Works make a game with Persona 5's unique style.
I'm not sure how big the roster would be. The eight "core" Phantom Thieves, plus the two who are added in Royal, the two who are added in Strikers, and the Velvet Room attendant(s) would make for thirteen playable characters already (assuming that Futaba is playable, since Rise was a non-combatant-turned-playable character in P4A - whether it's Caroline and Justine or Lavenza, I think they'd act as one character), which seems like a lot if they're starting from scratch. Having aged-up versions of Persona 4 characters would be great (given their ties to the police, Yu, Naoto, and Chie seem like a must if they're going that route), but maybe they'd save that for Persona 5 Arena Treasure Edition or something like that.
I haven't played much of Persona 4 Arena, but so far, the biggest issue that I've noticed is that, since the entire roster consists of good guys (at least in the original game), the reasons for them to fight in story mode seem rather questionable. (for example, both Yu and Yukiko know that they're being manipulated, have a rough idea of how they're being manipulated, and yet they fight all-out anyway) Maybe a conflict between the Persona 4 cast (who act as an Investigation Team) and the Persona 5 cast (who are the Phantom Thieves) would do a better job at justifying it, but that seems like it would balloon the roster by quite a bit. (bare minimum, that's another eight or nine characters, depending on whether or not Marie counts - I haven't played Persona 4 Golden)