Hendrix - setting the stage for the 22 century. He would be 70 this year.
John Lennon - not sure what he would be doing, but I would be listening. 1/2 of double fantasy was awesome. 1/2 of it was Yoko. In the world of Venn Diagrams, those two did not overlap.
Lowell George - Little Feat could have kept recording well into the 90s.
Frank Zappa - I cannot even begin to imagine where he would be now.