Do you think that Durge would be satisfied with single world ...
Or do you think that s/he would eventually turn their gaze towards gods ... eventually target his own father ... and maaaaybe, just maybe, in cruel twisted irony cause his own Durge Folly?

I imagine Durge would be like a plague upon everything living. Spreading their splashes of red until nothing remains. And when everything is dark and silent they would end the final living thing.. themselves.
Unless a hero down the path stops them, although they would have to be powerful and stop them before it is too late. I don't imagine the level 12 to be the cap for the Durge (or any of the companions) after the game has ended. Surely they would continue to build power.
I think if you accept Bhaal you essentially become a reincarnation of him. At least that's how I understood the ending. Can Durge eventually reach Bhaal and end his godhood? Maybe.. That would be interesting.
What if Bhaal was not the last on the list. I can imagine some other gods trying to stop Durge and a great battle. Maybe a stab in the back. Some gods taking advantage of the shift in focus to end their rivals. But would they manage to end the Durge, the plague before it's too late?