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.
I'd agree if it wasn't changing itself up so much in general this coming year, with the introduction of 3 more stages covering various genres a return to a "Metal Day" might be a great thing to do/something they may encourage.
If The Vaccines are there again, I'll be surprised/bored for sure. They were main stage and fairly high up last year, with nothing new album wise by next year, that would be a bad booking.
A much as I enjoy the album, and want to see a full set, alt-J as a headline on NME/Radio 1 stage is too much this early in their life as a band.