I'm actually fairly ambivalent towards Coldplay, saw them live a couple of times on the AROBTTH and X&Y tours and they put on a good show. Their musical output has been rotten for a good few years now but had they not been up against LCD I'd have probably gone along to see them.
All that being said it's definitely too soon for them to come back. I'd be pretty disappointed to see any retuning headliners from the last few years, exception being The Stones as a) its been just long enough b) they'd be perfect for the anniversary.