Such a great series. I've been replaying some of the games recently actually. I never finished AAI, so I went and replayed the entire game up until the last case (the new Layton got in the way, so I'll finish it soon-ish after that). I also forgot a lot about what happened in the last case in AA4, so I replayed that case as well. The Video Games Awesome! crew are doing an AA1-3 playthrough, so that's great to refresh my memory about those. It's all working up to the release of AA5!
Very excited to see who'll return in AA5. Maya needs to be there, at least for an important appearance and they can't just ignore Apollo. Also it wouldn't be a PW game without Larry Butz, Maya/Pearl, Edgeworth and Gumshoe.
But therein lies the potential problem of a new PW game. These characters are all so iconic and have many fans, but to put them all into one new game would probably be overkill, but if you don't the fans will revolt. The best way would probably be to give each of these a supporting role in any one case. Then the fans will know what became of these characters all these years later, but they don't have to impact the main story or overshadow the new characters in any way.