This is a very interesting thread about something I've been thinking about a lot lately.
I agree with many people here that JR would want to retire from touring because of his age. I would say he stays for 2-3 more albums/tours before he decides to leave in an amicable way. I feel like he would focus on his solo career and app developmennt, as well as spending time with his family.
The other posibility, and one that I would like to happen, not because I dislike the guy, but because I think a change on this area could benefit the band, is James leaving.
I really like his voice and he was perfect on TA, but, tbh, he struggles a lot live and I would like to see how DT sounds with a different vocalist (not you, WDADU or Majesty demos).
About Mangini leaving, I don't think it's going to happen anytime soon, but, as some of you have said, he doesn't seem to like heavy touring too much.
Anyway, if he decides to leave, I don't think the band would ask Portnoy to return, the would get another drummer, imo.