There's a couple things I can think of that are fitting as buffs to barbs. Keeping in mind that they are savage, primal warriors, fighting on instinct and fury rather than technique and training.
#1 - When raged, gain some amount of armor penetration. The fury driving the barb's massive blows is simply far more force than can be readily handled. (perhaps 1% fort reduction per barb level, with Ravager getting some enhancement to gain more?)
#2 - Every point of barb DR should also provide 2 PRR, further reducing inbound damage. Occult Slayer can add MRR as well, perhaps in the cores.
#3 - Steal some of the Rashemi stuff. Let the barbs pick a 'totem' for their fury, that gives them a new action boost. The totem has to be something found in the wild, known for it's fury and endurance. Examples - Troll (regen action boost), Dire Badger (Immunity to Knockdown + freedom of movement for a short duration...the furious badgers don't stop coming!), Dire Wolverine (attacks cause stacks of bleeding for the duration of the boost), Raptor - yes, as in Velociraptor (pounce! Leaps onto the target, knocking them down for 3-9 seconds. Has a minimum range.) Dragon (+10 str, con, AC, +20 PRR & MRR, 30 seconds per boost - dragons are renowned for their power and endurance). Could even make those boosts only work when raged, but that could be restrictive since there's a limited number of rages.