I think the most fans can hope for is a reunion of LTE. Even if MP mends whatever issues there may be with James (and if some exist with JM and MM), I don't see DT having MP as a guest. That opens too many questions, and honestly...would be a bad business move, considering all the time they've put into making MM significantly part of the band (in the eyes of both fans and industry). While I'd never say never on MP guesting with DT, I just don't find it to be likely at all.
But LTE reuniting, or JP/JR showing up at one of Mike's gigs to do a couple of DT songs? I'd find that very possible, and likely, moving forward. Just my gut feeling on it.
MP and JP were friends and bandmates and business partners for too long to let that relationship be ripped apart. But as long as MP is comfortable knowing that DT is not something he'll be a part of again, I think the two of them move forward as brothers.
The only scenario I ever see MP rejoining Dream Theater again is if MM decides, on his own volition, to step down. In that case, given that JP and JR drive Dream Theater, I absolutely think MP would be the first person they call. And if bridges haven't been rebuilt to JL and JM by then, I think it happens anyway. But again, that's if and ONLY IF, Mangini voluntarily, on his own volition, for his own reasons, leaves the band. I don't think Dream Theater will ever ask him to leave. They like MM, MM likes them. It would have to be MM deciding that he needs a change.