It's less a question for me of "what" and more of "quality". P4 is along with Trails one of those that should have lifted up other light JRPG narratives, not be at risk of being drug down to weaker one's level. If I wanted that sort of thing, I have far more options to choose from.
Really, Persona 5 should be their best game. The realist in me says that it's very unlikely that P5 will be tonally similar to Golden and Arena, especially considering the extremely high quality of Catherine's writing (Easily their best game yet in that regard).
I think P3P showed how much they've learned. S Links had some small bearing on the story, Shinjiro's S Link included the full party in an unobtrusive way, Saori's S Link was incredibly depressing and serious and by this point Soejima is deliberately going out of his way to design fashionable, cool characters.
Golden and Arena were something to tide people over whilst the big boys worked on something meaningful.
Of course I could be wrong and robot girls and hot springs and kawaii are all that awaits us.
I shall stick with my original beliefs and claims. Persona 5 for the PS3/Vita. Believe!
Still say there's no chance of a Vita port on day 1.
No chance.