Atlus has kind of gotten in an "All In" position with this whole thing. They've spent good money on promotion for it, and then with the running of it if all of this new VA work is any indication.
I cannot possibly imagine a scenario right now where this *isn't* Persona 5.
Can it be a Persona 4-2 as well? Sure. Do I have faith in them to not make it suck? Yes, because they've earned that right. They've both kind of written themselves into a corner with the Arena storyline in terms of what it can be....or they could just turn around and say "screw it" and do their own thing. Personally? I'd LOVE for 4-2 to be a competent RPG re-telling of the events of Arena, starting with Labrys dungeon and made into a full fledged sequel. I'm also ok with the idea of it taking place a decade later with a new MC, while some familiar faces have now taken on their roles around Inaba.
I'm thinking Persona 5 is a definite. I still think Persona 3 Vita is happening due to the movie timing with this(since the P4 Animation timing synced with Persona 4 Golden), plus the countdowns have been more P3 heavy than P4. I could see a P4-2 as well, though I think this is the least likely of the three. The only thing that genuinely bugs me about this whole thing is them pretending P1 and P2 just don't exist. You'd think when you have to fill in many hours of fan service stuff, you could tell the stories of P1 and P2 instead of putting up another Marie poem.