I think it depends what kind of venues they're pulling for. If they're playing the same size venues as 2012 then I don't see Fear Factory being unable to pull in people as a headliner. I mean DevilDriver headlined this year and FF is a lot more popular than DD.
05/06 - Misery Index
07/19 - Rise Against/Killswitch Engage
08/08 - Slipknot/Lamb of God