Dream Mash-Up

Many beautifully talented musicians have died young with such wisdom (John Lennon and Jimi Hendrix for example) and I can't help but imagine which artists would jam the best and send the largest message; have the biggest spot in history; create music that you'd listen to everyday.  Which artists, living or dead, would you want to "come together'' and jam out a new album?  What would the album be about?