I personally really want a general ATLUS travelling stage - including the streets Tokyo from SMTIV and SMTIVA, Destinyland from SMT1, the Idolsphere from TMS#FE, a labyrinth from Etrian Odyssey, etc., though I agree that the Velvet Room would be best for Persona in particular - maybe including five versions, if they want to use the P1/2 and PQ designs in addition to the iconic ones.
In terms of assist trophies, Morgana, Teddie, and Aigis would make sense as Persona mascots. Jack Frost/the Jack Bros would make sense as ATLUS mascots as well. I could also see Flynn, Nanashi, "Hero," Demifeind, etc. be chosen as ATs if mainline MegaTen wasn't being represented directly.