Well, I always liked taking four levels of fighter for the weapon specialization. The extra fighter bonus feats can make up for the levels taken in fighter, so you don't lose much at all except your highest barbarian class abilities. You can take fighter anytime during your barbarian career, but why not get to read/write a language early on as a nice extra benefit too?
I like ranger as a good alternative if you have a favored enemy in mind. I see tracking as an important barbarian feat too--how else would they survive in the wild if they couldn't hunt? Anyway, the class abilities mesh better with light/medium armor too, but you won't get the bonus feats of a fighter or it's weapon specialization.
Any kind of spellcasting class is just not for you, as your stats couldn't support it. So, don't forget to shore up your WILL save with the Iron Will feat whenever it is convenient for you. Or make sure you rage whenever you see an enemy spellcaster. You'll find yourself vulnerable to spellcasters outside of battle, and without your rage, so don't be shy about keeping your distance from them unless necessary.
*edit*
As a higher level barbarian, you may get more rages per day, but how many fights do you really get into in one single day? If you know there will be several fights during the day, just conserve your last rage for the most important fight. I think more than three rages a day is really pointless.
The additional DR is a nice trait for barbarians, but not really necessary considering barbarians already get d12 HP/level. I'd rather go multiclass with ranger or fighter.
*edit again*
As for ability points, if you don't have your heart set on strength, I would go for increased DEX if you plan to stay with light armors, or a couple points in CON if you decide to choose heavy armor, as it will solidify both your HP and your already impressive FORT save. Those higher level encounters often mean saving throws are just as important if not more so than AC. A fighter's weapon specialization is more economical to increase damage.