Well, regardless of what happens, I don't think there's any way a sequel to P4AU gets announced within two years from now. I think at this point, "P4A" is really just a brand, and maybe they'd still keep the name even if it featured mostly a completely new cast or P5 characters.
Considering that, if P4U3 was to be announced in 2016-2017, I think that could maybe give P5 enough time to establish itself. If not, then like you said, it would either be the path of including P1/P2 characters or a completely new cast, if we were to assume that there would be a sequel and it wasn't going to be announced 4-5 years from now. I'm not too sure a sequel is inevitable, though, even though Wada would obviously want one.
Edit: Yeah, from a creative standpoint, it's just so hard to imagine a sequel right now that wouldn't be P5 characters added in. Practically everyone from P3 and P4 is in there now. I really don't think P1 and P2 will ever get into the spotlight again. A new cast is a possibility, but like Rhapsody said, I don't think it would be a profitable venture for them given the "hype" factor the two games in the P4U series have had leading up to their release.