I think the first step you should take would be to identify what you don't like about the barbarian.
Myself for instance, I don't like
1- Its a 1 level Dip
2- Its a fighter who gets angry
3- They wear armor, and use weapons. Often Full plate (mithril) and greatswords.
4- Not actually very nature themed.
So I would try to include things like
1- At level 1 you get "Primary Ability" This ability grants a benefit that scales as you take levels in this class. This scales regularly like every 3 levels or so. Along with Abilities gained at levels 2, 3, 4, and at least 5 that are solid "Hey thats cool, I want that".
2- No armor or weapon proficiency. Bonuses to unarmored, and unarmed. Maybe an unarmed strike bonus, improvised weapon bonus (That i would let transfer to untrained weapons), unarmored AC and/or DR bonus,
3- I'd include nature totems inspired from complete champion, and unearthed arcana. Stuff like "Wolf" that gives trips, bear that gives grabs, eagle that gives flight (or jumps), snake that gives poisonous bites, cats giving stalking pounce powers. These would probably kick in at level 2, 5, 7, 10, 14, and 18. Each time it would give a new thing they can do.
Again I want my barbarian in a loin cloth, and ram horn hats ramming people. Then when the foe goes down, they grab his weapon and just bash people through sheer strength regardless of their opponents weapon skill.
You could totally want to play up the nature aspect though, and have people who are more "Super dex guy" who runs up, is impossible to hit, and jumps on his foes back biting them in the neck. I'd want that to be a legit option and career choice for my barbarian class. Maybe something like "You get an Animal Trance. It gives you + {Barbarian Level} to a stat of your choice as an enhancement bonus (Helping them shy away from items, which I also would like my barbarians to do). Once this choice is made, it is permanent. While in this trance, you cannot cast spells, or do normal barbarian rage restrictions. You can do this once a fight triggered by X, Y, or Z" instead of rage. X, Y, or Z could be "as you deal damage, as you take damage, so long as you have taken no damage, as you miss an opponent in melee, as you dodge an attack in melee, as you are targeted by a spell," and so on.