What Could Have Been: The never-to-be-seen "HELP", with Jimi Hendrix as the guitarist. Hendrix died before ELP could have a proper meeting with him.
According to Keith Emerson in the book "Pictures of an Exhibitionist", this was a not-entirely-true rumor. The way he described it, when he and Greg Lake were looking for a drummer, they at one point considered Mitch Mitchell (formerly of the Jimi Hendrix Experience), who mentioned to them "Maybe I could get Jimi interested in joining", but that's as far as it went. Jimi was apparently supposed to try jamming with them, but he died first.