Episode P4 will remain incomplete until you've finished Episode P3. I preferred Episode P3 to Episode P4, but it's not as if the story fundamentally changes; it's the character moments that make it.
From this interview. Wada states that P4U2 was a sequel to P4 as well as the "P-1 Grand Prix", the latter of which has concluded with this entry, and I believe he also states he'd be willing to explore a future game. He stated his willingness for the P4A series to be a true fighting game series known among the popular ones out there in the P4A art book interview before, too.
Statements like that, the success of the series, P4AU's true ending, etc. makes it so that a sequel would be likely. Although, I still have to wonder what they could do with it, especially narratively. Even in terms of roster, the only characters they could really add anymore are Theodore and Fuuka, and those are some real borderline choices right there. We can consider the P5 crew, but then will they have to ditch a lot of the sprites of the P4 cast, or is something silly like time travel going to be involved (assuming P5 takes place years in the future from those events)? It's at a weird place, when things seemed pretty obvious after P4A.
