If you're not willing to give Barbarians the DR or PRR which make the most sense for their class, what about an ability equivalent to the Invincibility power orb in Challenges? I'm thinking spend all your Occult Slayer Bond and get 10-20 seconds of taking 0 damage from all sources, and then a looooong cooldown, plus the need to build Bond back up again. That would at least make them feel more powerful in fights that matter. You could even make it a 1-shot power from tier 5 of each of the three trees that only resets on rest.