As much as I love Phoenix, I have to agree with you on that. I'm not all that attached to any AJ characters (except AJ!era Ema Skye, who grew up to be an amazing, deadpan sort of hilarious) and I think I can attribute that, in part, to how the game treats even its main character. If they wanted to keep the series about Phoenix, and the plot centered around him, they could have done that. Apollo got the short end of the stick. I admittedly don't like AJ!Phoenix as much as the high-strung, inwardly sarcastic, deeply motivated Phoenix from his own games in the first place.
Don't get me wrong, though, I enjoyed the game deeply! Even with my objective acknowledgement that it should have let Apollo have his own story, I still have a deep, abiding love for the flashbacks.
I liked Ace Attorney: Investigations, too, but not quite as much. I love that you allude to its differences, because it makes me remember all the jokes people made about WALKING! when it first came out. A new, innovative feature - unprecedented: ACTUALLY WALKING AROUND! I do think a game about Edgeworth shouldn't have been kept entirely out of court, though - they could have made the same game with Gumshoe as a main character, and still have Edgeworth feature in it.
Perhaps I'm biased, though: everyone seems to love Edgeworth more than me. I don't dislike him, but I don't love him either.
I confess I needed to look up who David Cage is - I loved Heavy Rain, but I don't think that sort of hyper-realistic style would translate well to a franchise based on goofy character designs and mostly sprite-based art. But, then, the live action movie was (incredibly campy and) very well done, so who knows?
One caveat, though: Nick Cage has to play everyone. Nick Cage as Phoenix. Nick Cage as Edgeworth. Nick Cage as Pearls channeling Mia.
I can't wait for the cutscenes, either! I've waited so long for any sort of animation - perhaps an animated movie/show is still within the realms of possibility. I can dream.