Pulp and Chemical Brother's have only headlined recently so I'm not entirely sure they'd be back, I definitely think Biffy stand a chance though
There's rumours that The Cure are releasing a new album soon which would probably mean they'd tour too. I know it's a pipe dream but they seem the definitive band to have never played
I also listened to the Florence and the Machine Prom the other night and would give anything for her to perform it at the festival, James' orchestral performance was a resounding success and I'd live to see it done there too