What if each of the new bosses dropped a piece of an item that when combined with the ones from the other bosses creates an item that lets you imbue the brimstone ring? Could also make either the parts, or the item itself tradeable. Maybe it makes the upgraded brimstone ring untradeable? So it would be a permanent unlock like the avernic. Or any combination of tradeable/untradeable. I'm just spit balling here.
This would cut down the power creep from the content and implement something that is both quality of life and a reasonable upgrade.