If there's gonna be Ultimate Personae, there's gotta be a good set of conditions for that, key Personae we can use for Fusions (Provided the Fusion mechanics will be used here), have your character on high levels for the upkeep (You can't use one if your level is too low just like you can't summon a demon from your COMP because of this) and do so from sidequests. I think this could be reasonable if you want to include Ultimates.