Male vocalists:  Dylan of course, and Lightfoot..of course, Lightfoot (natural match), maybe Steve Earle (think about what a wild version of Hell Bound Train they could do)...

Female: First choice: Karla Bonoff   Then maybe: Emmylou Harris, Tracy Chapman, Vonda Shepard, yes, Joni!, Rickie Lee Jones (did he do that already?), ..I don't know..how about Hazel Dickens?  

Great thread...let's hear some more choices.  Then...let's hear from the man himself !!!   I'm very curious...Who would HE like to sing with?

Gene