Honestly, I can imagine the remaining four characters be one or two smaller/niche third parties, one more 1st party, and one more big third party. At the very least that is how I see it.
Funnily enough I'd be surprised if we don't get a lesser known/niche/"small" third party choice, or at least one people have written off. A Terry-esque pick or two would add some nice diversity to the list. This is why I haven't written off characters like Sol Badguy, Reimu, Monokuma, etc etc yet. Travis could also likely fall under this.
A 1st party rep I can not think of much except Rex/Pyra or Bandana Waddle Dee.
A big third party could be someone like Dante or whatnot.
This is just my little personal guesses and I am likely wrong but it feels like a big of a nice balance on things.