kooks are pretty mainstream lad. listen to the first album: more or less every song was a hit single, and deservedly so i must say. even though i don't like them, i've fot to give it to them they know how to write a pop song. That Shine On song from the second album was a bit shit though. So yes i'd argue they have both the required level of talent and MOR appeal, and would say it was possible they'll be there. hope not though
i have a strong feeling about morissey and doves. keane was a good suggestion too. cure and arcade fire would be amazing but i don't think either will be there, personally: the former are too big, the latter headlined too recently, and are out of festival action for now... oh, and the flaming lips...now we can hope... one other dream...biffy clyro for headliners...?!