I am still of the thought that "New Prosecutor" is the main prosecutor for the Japan/America side of things, while there has to be someone else for the Kurain side of things.
Case 1 = Kurain = Payne
Case 2 = Japan/America = Nayuta (looked it up)
Case 3 = Kurain = ???
Case 4 = Japan/America = Nayuta
Case 5 = ???
I refuse to believe Nayuta is traveling between Japan/America and Kuraindespite being labeled an "international prosecutor"between cases. And I also do not believe that, if there were to be another court prosecutor, they'd be yet another new character, especially not one with the mysticism theme Nayuta already has.
So they'd be a returning charactermost likely someone who was already a prosecutorand Franziska is the one who's known for traveling to different countries and who has also had experience with the mysticism stuff.
Could be Edgeworth, but then I'd really like to know what the hell is going on at Kurain Kingdom for Phoenix Wright and Edgeworth to find themselves there, while Nayuta who is a resident of the place decided to travel to Japan/America while they were away.
Hmm, ok I have to admit you present a very good point.