The way I see it, this tour is only getting bigger so any bands that are less popular than Cannibal Corpse and BTBAM are unlikely to headline. CoB sounds the most likely of all the names being thrown around. CoB and Amon Amarth would be sweet. Machine Head would be weird because they don't really count as an extreme metal band.
6/13-Iceage or Chelsea Wolfe/Swans
6/29-Lil B 'The Based God'