I was thinking about this the other day, and something popped into my head. Rather than have the incredible number of starting HPs (18 seems really high), why don't you make it a feature of the rage. The 'fun' part of playing a barbarian is flying into a rage and beating on things, so it makes sense that a player will want to rage pretty much every encounter.
I was thinking this might help fuel some of the rage powers, since they cost HP.Class Feature: Barbarian Rage
Once per encounter, you can tap into your primal instincts and fly into a rage, becoming a howling juggernaut. You gain temporary HP equal to your level plus your constitution score. The rage lasts for 5 minutes or until the end of the encounter.