I've been hoping they'd make it back to Coachella the last few years but no dice. As far as headlining White kids wouldn't know enough Roots or Soulquarians songs for them to headline. Dr Dre & Snoop & friends have songs that every white kid knows even if they don't listen to hip hop. As far as other hip hop groups that haven't already definitely Outkast as you mentioned, Wu-Tang (If all the main members where all there and maybe a ODB hologram lol), MAYBE A Tribe Called Quest, not actually a group but if The Black Keys did a BlakRoc set with a bunch of rappers it could headline but doubt that could come together.