I feel that Phoenix's story should totally have a ton of new characters and maybe Maya. New music, new mechanics, new setting, new Rival Prosecutor. They can have Psyche Locks and the Water Mirror.
However, I want to see a ton of AJ stuff for Apollo's segments. I want to see Trucy, Klavier, and Ema be involved. We know Athena will, but Trucy and Athena can be balanced. Ema can be the detective, and Klavier can be the prosecutor. Remix everything for Apollo, unlike for Phoenix. Remix his Pursuit and Objection themes and maybe some more. I'd like to see some DD characters come back. Juniper seems like a must have. Maybe Jinxie. They can have Perceiving and Mood Matrix.
this seems like a best case scenario tbh