I just think that for an SB halftime performance to be memorable, there has to be a "WOW" factor from a visual standpoint and not just be the band standing there playing songs. For example, during Prince's set when they dropped that huge white curtain and it was a silhouette of him playing his guitar....that was just cool to look at. Or with U2 - forget the screen with all the 9/11 names, the coolest thing to me was the stage setup (which they kind of used again on the Vertigo tour) that allowed Bono to sprint out into the crowd and be surrounded by the masses of people. If he had just stayed up on the main stage with the rest of the band members for the whole set, it would have been boring. So, in essence, I think you need the show to look cool and it also helps if the band is young-ish (comparatively speaking - i.e. U2, Prince are of a different generation than Rolling Stones and McCartney) and they interact with the crowd. I think there are a number of bands that could pull it off - Foo Fighters, The Killers, Coldplay, Red Hot Chili Peppers are the ones that first come to my mind that would have the broad popular appeal necessary to headline.