Like... anything? Why would you ever take 10 levels of barbarian? Two levels of fighter for feats, 1 level of cloistered cleric for Travel Devotion (if you didn't take the 1st level barbarian substitution level that grants pounce for some reason...), a few ranger levels for skills... seriously, just about anything is better than taking 10 levels of barbarian. I guess the only advantage for more barbarian is that it stacks with Runescarred 'Zerker for some things? I can't remember if that's true. Anyway, for more rages, just take the Extra Rage feat.