If you're fine with homebrew, I'd recommend this PRC: http://www.giantitp.com/forums/showthread.php?t=113692. It's currently the only way outside of great gobs of druid or druid-like levels to get a third shifter trait, plus a goodly number of bonus shifter feats for boosting your shifting further.

As a Shifter Barbarian, I'd heartilly recommend having your first shifter trait be Beasthide and going Beasthide Elite, so that a) you have more staying power when you rage and shift, b) your AC actually goes up while shift/raged and c) OMG hit points. If you insist on a attack trait, I suppose recommending the powerful charge gore trait would probably be the better choice, as it does decent damage on that charge, plus tripping, IIRC. After which you can start hitting things w/ your favored two-hander.

There's also a rage/shift feat in Races of Eberron that maeks your natural attacks pretty devastating, IIRC.

Them's my 2 coppers. Take as you will.