John Petrucci's got years of G3 ahead of him. I assume he'd amass a small solo band, too. Maybe not even solo, just another band. Continue writing music, tour the world, just do his own thing with some different people on board.
Mike Portnoy will occasionally moan about it on his website (KIDDING) and do a hell of a load of collaborations. He seems like the kind of person who would want to work with as many artists as possible.
John Myung will briefly join Pain of Salvation, before inevitably leaving again (as all PoS bassists do). Then I could imagine him going into something less ostentatious. Tutoring, perhaps. (Specifically bass tutoring.) A couple of quiet sideprojects but he'd mainly fade into obscurity.
Ol' Jimbo will try and join Broadway. Musical theatre, I reckon. He's said he'd be interested in it, and I figure he'd probably take that opportunity and do something theatrical. He'd maintain a healthy and open relationship with the Dream Theater fans and be more than happy to reunite and do interviews and such, and possibly do the odd sideproject or join in with one of Mike's collaborations perhaps, but by and large we'd be talking about "James's Broadway Career."
Jordan Rudess will be heartbroken, and found dead in a hotel room, having drowned himself in a vase of flowers. His iPhone was also found in the vase.
Derek Sherinian will be involved in the reunion tour, circa 10 years later.