Personally, my feeling on the Black Council's goal is to destroy the current order of the world to establish a new one. That usually implies that they'd be the ones at the top of that hierarchy. So less about bringing the Old Gods back and more about becoming Old Gods themselves.
I've also toyed with the idea that they're trying to help the Outsiders destroy this reality so that they can create a new one, specifically one that meets their idea of what it could or should be. In that reality, death can be impossible, for instance. But ultimately, there wouldn't be much free will in that reality, because there wouldn't be many meaningful choices, or consequences for the ones you could make.