Just a quick suggestion, inspired by zDark Shadowz's Unchained Rage thread:
The fact that Barbarian Rage increases CON temporarily has always been a rather sore point. Having rage fade when at low health has killed many a barbarian.
So, I propose a simple change: Instead of giving CON, have it give the Barbarian temporary hitpoints that last until the rage fades.
Regular Rage : 2 * Barbarian Levels
Greater Rage : 3 * Barbarian Levels
Mighty Rage : 4 * Barbarian Levels
If you have reckless rage, add +1 to the numbers above.
You get the same extra points as the CON would, only they're the first hitpoints subtracted.
Could you still stack extra CON on top of that and get more health? Sure, but you're able to easily lose it when dispelled/fading, which can have the same disastrous effects as Rage fading.