I know everyone wants Maya back and all, but...I'd rather she didn't make a big appearance. The first three games were about the Fey family, and I think that the fact that they've been keeping their appearances to a minimum was a very smart decision. It makes it feel less gratuitous and less forced, really drilling in the the fact that she's in the position she's in and that she's a busy woman.
If they bring her back at all, I'd like it to be at the end of the 3DS trilogy (because, and I'm not sure if this is intentional or not, but each system has had three Ace Attorney titles (GBA with PW 1,2, and 3; DS with AJ, AAI 1 and 2; 3DS with DD and whatever comes next) it's just speculation on my part, though), either as an endgame appearance or a trial based around the Fey family.