I was thinking about this last night, and I didn't see any threads on here like this, so I figured I'd give it a shot.
Just make a band with whatever members you want.
I limited mine to one member from a band, but do whatever you want.
Here is my "Extreme Progressive" Supergroup.
Mikael Akerfeldt (Opeth/Bloodbath) - Guitar/Vocals
Ihsahn (Emperor) - Guitar/Vocals
Grutle Kjellson (Enslaved) - Bass/Vocals
Sean Reinert (Cynic/Death) - Drums
Janne Wirman (Children of Bodom) - Keyboards
I would have Mikael, Ihsahn, and Grutle all sharing vocal duty, kind of similiar to how Mastodon does it these days. They would all do clean vocals too (Think of those harmonies!!!). Couldn't really think of what keyboardists I could have put in here, so I just went with Wirman from CoB.
If anyone else has any ideas put them up! I'm sure they'll be interesting.
I've lost track of just about everything.