I agree that Kaiser Chiefs are far more likely but has anyone considered Biffy Clyro?
They're currently touring; there'a a glastonbury shaped hole in their schedule; they would be perfect before Foos; did a secret set in 2010 and played 3rd on pyramid to quite a big crowd in 2011; they're always doing the rounds but seem to have missed Glasto on the last 2 albums; and it would fit worth the "2 years worth of music in 1".
Ok they would probably want to headline but they seem to have been missed out by Glasto over the last couple of years and you could argue that maybe they're still not quite big enough for it? Especially as their last album was white average in my opinion and they aren't bonafide headliners elsewhere other than the UK. They also probably wouldn't want to do a shortened set, so Foos doing 3 hours maybe makes it less likely.
Just a thought!