Really ... :/ I think he's okay after his rookie/arrogant time, and I don't care for Gumshoe, so he doesn't bother me much. It is not as good as PW, but meh still hella better than AJ >=(
Since after Apollo, Nick retired, I doubt there would be any comeback of the combo Nick-Maya-Pearl -_- And because Nick retired + the second (seemingly) most main character - Edgeworth - has already had his own game, I don't think there's a need for a wrap up. Well, plot wise, anyway.
The only ways I could think of for any new game are:
1. flashback of past cases (then Nick may be able to reappear. That would be lame but oh well)
2. cases of other characters (like with Edgeworth)
3. new characters (like with Apollo - this would very likely make me rage as hell unless the new character is super super cool <_>)