Let's see, a final boss...
A logical choice for final boss would be Manfred von Karma, who is an established villian and who has had an indirect 'presence' in most of the games through his daughter. But he'd have to prosecute from jail (or, assuming he was executed, if someone was stupid enough to channel him, which seems unlikely.) von Karma also has a strong connection to almost all the main characters in one way or another.
I suppose Gant could return, but he doesn't have any connection to anything outside of his one case... it would be a lot of work to tie him into them. Also, people who have been sentenced to death like Gant and von Karma don't usually come back in this series, although they might make an exception.
Pearl's mother still swore revenge and is still alive, so she's a possibility. She also featured pretty importantly in the second and third games, and has important connections to several characters (although not as many as von Karma).
If there's going to be a big bad evil behind everything, it's logical to try and pin as much on him (or her) as possible, to try and tie everything together. Making him responsible for all those cases that went wrong in the past, making him responsible (somehow) for Mia's death, etc. etc.
It's also possible that the 'final boss' isn't physically present (or alive anymore), but that the last case deals with something they set in motion in the past...
