Music Collaborations That Should (Or Should Have) Happened

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Should Have

Yoko Ono and Frank Zappa

Phil Lynott and Rory Gallagher (a pair of Irishmen of approximately the same vintage who played Blues based Rock 'n' Roll and both drank themselves to death.  As far as I know, they never collaborated).

Should

Yoko Ono and Devo

Phil Lesh and Billy Strings (they can call it the Philbillys)

More of an artist-covers-artist what-if thing, but I've imagined Johnny Cash singing Brandi Carlisle's The Joke.

In dropped-D.

Miles Davis and the Grateful Dead - April 1970 (almost)

The original (unnamed) touring "Garcia Project" that was at least casually in the works and was slated to include some regulars and other rotating artists with names being kicked around such as Bruce Hornsby, Bonnie Raitte, Edie Brickell, maybe Paul Simon, etc. with no defined set lists or even musical styles. Jerry was fairly booked and busy but that project would have been something to see had it ever materialized.

Miles Davis and Jimi Hendrix. It was supposed to happen.

Garcia and Dylan acoustic guitars

they both had such a vast knowledge of old songs