From what I've heard, there won't be a return to a "metal day" for several reasons. Firstly, it doesn't really go with the rest of the festival, and secondly, the emergence of Sonisphere and Hevy, alongside Download, mean that metal festivals are pretty well covered.
Obvious acts are TDCC, Twin Atlantic, Lower Than Atlantis, Deaf Havana, The Vaccines, Jake Bugg and Florence And The Machine. I would expect most, if not all, to be on the bill this year.
Biffy Clyro will probably headline one day.
True but it is worth taking into account that Sonisphere died on its arse this year, and Hevy is more Lock-Up type stuff than the sort of really big heavier bands who'd probably be high up on the main stages.