Steven, Mikael, Portnoy and Devin would never actually agree to form a supergroup, but if they were absolutely forced to for some reason, I imagine it would go like this:
Steven and Mikael, being good friends, past collaborators, and the most similar in musical tastes, would immediately take the reigns and do whatever they'd want to do. They'd probably come up with something with a weird combination of influences that doesn't sound anything like conventional prog or even like Storm Corrosion (they've said if they were to do a follow-up to that album it would sound completely different). Portnoy would inevitably try to steer things in a more upbeat, proggy, or metal direction and likely get shot down by Steven and Mikael. In a repeat of the OSI sequence of events, he'd eventually give up, and in future interviews claim Steven and Mikael were "downers" and "difficult to work with" and that the project wasn't "what he hoped it would be".
Devin would be there. He'd play bass, sing on a few songs, maybe contribute a few musical ideas, but generally just sit back and let everyone else do their thing.
The album would be a disappointment to most people but it'd probably be quite good in its own weird way.