>>Barbs could go and kill a couple dozen enemies to get bonused BMR for a few hours.

Please no.

I don't want BMR to work like Thief confidence. Confidence works fine as a bonus system in conjunction with urban bonus but BMR has always been very dependent on inner fire.

I think it is a very good idea to throw out all current preconceptions about how MR will work for Barbarians to come up with the best possible solution. I also think it's important, however, to decide which aspects of BMR we want to keep. Completely changing its scaling or functionality works fine, but the relationship between IF and BMR strikes me as a very important theme.
*******
Because it's a gigantic can of worms. Eldritch, necrotic, squamous worms, writhing in a vile stew of coagulating ichor, crushed from the living gullets of a thousand infant puppies, ululating in wordless terror.