Maneuver access.
They don't actually need that much, Barbarians are good at breaking stuff with a big weapon. It's just all the other things.
Anyway, what kind of barbarian, first and foremost? Does play style matter?
If play style is to remain largely simple, then you'd want to work on the passives:
-Temporary hit points/damage reduction/energy resistance/spell resistance. Self-recovery abilities(hp, ability damage, conditions) General being tough stuffs.
-Passive bonuses to 'action' skills, like changing Jump distance formulae(so you can actually get anywhere by jumping), etc.
-Effective size enhancement. Not just damage, but the reach.
-Get high level staples via supersized skills, pseudo-flight especially(for the rest they can piggyback, it's not strictly necessary on a daily basis).
Basically just kick the shit out of everything at maximum force.
If you want moderate complexity:
War Frenzy. Actual selectable abilities and broad based capabilities, but overall not too complex to run or build.
If you want high complexity, then it'd be rage power options, everything can be made stronger by incorporating a casting mechanic, but consider what you're losing in the process from the play experience of a barbarian.