As the tin says, here you may post some counterfactual band lineups. Whether rock, jazz, or such, as long as its post 1900.

I'll start:

Hendrix, Emerson, Lake and Palmer (HELP)

A famous rock supergroup composed of Jimi Hendrix (guitar, vocals), Keith Emerson (keyboards), Greg Lake (bass, vocals), and Carl Palmer (drums). The band got its start after Woodstock, and after The Jimi Hendrix Experience disbanded, Hendrix approached Emerson (formerly of The Nice) and Lake (formerly of King Crimson) to form a band.

HELP went on to be international superstars, with Hendrix's stinging blues influenced riffs complementing Emerson's classically based playing, while Lake and Palmer supplied a pounding rhythm section that kept the band in lockstep with each other.

Nevertheless, HELP disbanded in 1977 due to creative differences, primarily between Hendrix and Emerson, and the disappointing performance of their album Sun Beach. To this day, however, HELP is considered one of the greatest progressive rock bands and are noted to be the forerunners of progressive metal.