The infernal sorcerer idea is a good one, but I would come at it from the other side, as well. There are a lot of situations--roleplaying oportunities--inherent in the idea of a divinely motivated arcanist trying to avoid "the Man". Ostensibly, the creature he or she made the pact with wants the soul uncluttered, and doing lawless things, even in the name of the greater good, might set them at odds with a patron. Just an idea.
I also like Nifft's idea about the barbarian as a savage. The feat should, however, be part of a society, or some such. I know that's approaching too lawful for barbarians, but all of the things I've ever read about the berserkers say that it was a cold and murderous battle-trance, and they learned it through training, rather than any natural proclivity. They were the "Bear Shirt Society", named for the bearskins they wore into battle, IIRC.
All of the things that the PHB says they could do, though, is pretty close to what the legend says, except the traps thingie--I think all of that comes from Conan--so if you change the flavor, I wouldn't change the mechanics much. The con and str bonuses fit right in, as does the will save, representing that they had little to no concern for their safety while raging.
To make things more interesting, you might increase their speed for each x berserker levels gained, but only while raging. Damage and hitpoint bonuses are inherent in str and con bonuses, but you might increase damage and reduce defense a bit more, again, while raging.
I'm probably re-hashing stuff you already know, so I'll go.