The upcoming Desert Trip mega concert is full of opportunities for incredible onstage collaborations. It's easy to imagine Bob Dylan singing "Like a Rolling Stone" with the Rolling Stones on the first night (as they've done before) and Neil Young and Paul McCartney have many songs they could do together, including another run-through of "A Day in the Life." It's the final day, when Roger Waters and the Who perform, where it gets a little tricky. They don't have a real history of playing together, though Waters would probably get a kick out of joining them on "Won't Get Fooled Again."
There's a stronger precedent for the Who playing with David Gilmour. The Pink Floyd guitarist sang "The Dirty Jobs" with them at their 1996 reunion concert in Hyde Park, which you can watch above. It was the Who's first real concert in seven years, and they were joined by Gary Glitter, Stephen Fry and Phil Daniels for a complete performance of Quadrophenia. Gilmour came out again to play guitar on "Love Reign O'er Me." Roger Daltrey was the MVP of the evening since he was in extreme pain all night long because Gary Glitter accidentally fractured his eye socket while twirling a microphone stand during rehearsal.
Whether or not he performs with the Who, Roger Waters' set is the biggest unknown of the entire Desert Trip festival. He's the only one of the six performers not on tour at the moment. When he last toured, he played The Wall straight through, though that's not going to happen again at Desert Trip. Expect him to break out a bunch of Pink Floyd classics, though fans hoping to see David Gilmour make an appearance will almost certainly be disappointed.