Can't believe we're beating this dead horse again.
I voted zero because that's the closest option, but I don't really think it's zero. It's certainly POSSIBLE that, if MM decides to leave of his own accord, MP might rejoin, but think the chances are minute at best.
The other thing to keep in mind is that, even if the other four band members and MP all wanted him to come back, they'd also have to agree on the terms. I doubt very much that any of the current members would want MP to come back with the same role and responsibilities that he previously had.
I don't know how many times Petrucci or mangini has said THIS is the Dream Theater lineup until the end and there's not really a chance of Portnoy coming back.
I'm not sure either one of them has EVER said that (or words to that effect). It wouldn't be MM's place to comment on that, and all I can remember JP saying is that the current lineup is the current lineup and it would be pointless to discuss future "what ifs."