I second Occult Slayer.
Barbarians are fine power-level wise at the moment, but very one-dimensional.
Occult Slayer sounds, to me, like a Barbarian build that forgoes some of the FB damage capability for heightened defenses against both magical and physical attacks (perhaps the PrE would do something like granting +3 to all saves, and a reduction of 20% of all incoming spell damage, as well as some smite-like effect).
Sometimes a FB would be better, sometimes an Occult Slayer would be, but there'd be some more variety to the Barbarian class other than 'do you have Stunning Blow or Toughness'.