Vocals: Robert Smith
Bass: Flea
Guitar: James Dean Bradfield
Drums: Dave Grohl

If they were to form a band, the world would explode.

This would probably be an outright crash and burn group but here goes:

Drums - Neil Peart, Milford Graves, and John "Drumbo" French (that's right, triple drumming)
Reeds - Eric Dolphy, Bobby Mintzer, John Zorn, Ornette Coleman, and Pharoah Sanders
Bass - Les Claypool and Charnett Moffett
Guitar - Marc Ribot, Keith Rowe, John McLaughlin, and Eugene Chadbourne
Piano - Uri Caine
Organ - Jamie Saft
Vocals - Michael Patton and Yamatsuka Eye
Miscellaneous - Cyro Baptista

Guitar - Dean DeLeo
Bass - John Paul Jones
Drums - Carmen Appice
Vocals - Jack Black

Vocals: Mike Patton
Guitar: Tom Morello
Bass: Flea
Drums: Nicko McBrain

another one:

Vocals: Chris Cornell
Guitar: Dave Navarro
Bass: Les Claypool
Drums: Vinnie Paul Abbott

Vocals- Weird Al Yankovicz
Guitar- Peter Frampton
Bass- Bootsy Collins
Drums- Buddy Rich

Vocals: Deborah Harry, Faye Wong and Janet Jackson
Guitar: Brian May and Courtney Love
Bass: John Taylor
Drums: Phil Collins

For some reason I see this working.

Vocals: Maynard J. Keenan
Lead Guitar: John Petrucci
Rhythm: Tom Morello
Bass: Flea
Drums: Meg White

Another band I would do is a crossover band

Band Leader, Singer & Mi-Solo Guitarist: Franco
Singers: Bob Marley
Elvis Presley
Michael Jackson
Pepe Kalle
Solo Guitar: Papa Noel
Diblo Dibala
Mi-Solo: Elvis Presley
Rhythm Guitar: Lokassa Ya Mbongo
Bob Marley
Bass: Didier Masela
Keith Richards
Animations: Djouna Mumbafu
Drummer: Black men who would have been the drummer for MJ's this is it tour.

This is one I've conceived of for a while.

Nas- Rapping
Buckethead- Guitar
Victor Wooten- bass
Skream- Production

Somehow, I can see this working...

MC 1 - Aesop Rock
MC 2 - MF DOOM
Producer - EL-P
Scratching - DJ Q-Bert