If I had to guess I think A7X would headline over Zombie but I don't think they deserve it.
I think we will see Mayhem/Uproar headlined by A7X, Zombie, 5FDP. I'd love to see AiC & Deftones thrown in the mix but neither band does touring festivals in the US that much, although it would be a pleasant suprise & a nice way to shake things up. I think Lamb Of God, Hatebreed & Cannibal Corpse are locked in for Mayhem.
I think KsE, B4MV, In This Moment, AtR, BMTH & Suicide Silence could play Mayhem, Uproar (probably not SS) or Warped.
9-05 Iron Maiden, Megadeth