I agree with New Phoenix being better than his older version. He's more interesting to me, because of his less straightforward personality. Also, I found it interesting that he's less focused on defending innocents and more dedicated to refining and correcting the law itself and the legal system now.
I don't really see why you'd bring him back as a main character. Apollo is a bit different than original Phoenix: he's more emotional, more naive, and a bit less sure of himself, but otherwise is quite similar. Also, they really didn't develop him as much as Phoenix was in the first game; they spent more time on Phoenix than on Apollo. They can do a lot with what they have, and they can add more.
Personally, Phoenix is better as a support role. For some reason, I see him going two ways:
Spoiler tags for space and for possible GS4 spoilers (just in case, though it shouldn't be an issue in this forum):
Spoiler: Detective/Private Investigator
Spoiler: Prosecutor
Course, they could just make him a defense attorney again, or even just leave him as a piano player/card player, though I'd doubt they'd really do the latter because the plot device reasoning is gone.
I do, however, wish to see some characters from the first thee games return, particularly some of the staples. More particularly, Edgeworth, Franziska, Gumshoe, Pearl (less Maya - as Master, she's probably not going to have a huge part) and some one-shots as well. New characters are fine too, though.
Also, I demand more
, 
, and 
Lana because there's a tie-in to Ema and I like it, Viola because an older version as a crime boss might be pretty cool, and Basil because I like her and that's it.
