There are a lot of people who Paul could do a duet with and they'd sound cool, interesting, whatever. Here they are.

Billy Joel
Bob Marley
Buddy Holly
Chrissie Hynde
Christine McVie
Elvis Presley
Eric Clapton
Jerry Garcia
Jimi Hendrix
Joni Mitchell
Julian Lennon
Lindsey Buckingham
Neil Young
Otis Redding
Ozzy Osbourne
Paul Simon
Peter Tosh
Pete Townshend
Ray Charles
Ray Davies
U2
Yoko Ono