In terms of continuing the lore, one thing I thought was intriguing was the hellfire gambit Mephistopheles was working on. He was always my favorite archdevil. What if it worked, and his mortal cult was in ascendancy, but was then ruthlessly co-opted by Asmodeus, increasing the Lord of the Nine's visibility (the whole "is Asmodeus a god?" question starting to lean towards yes)? Mephistopheles was regularly confused for his boss anyway, and losing most of his cult after working so hard to build it up would be infuriating to him.
Mephistopheles, to me, was always the Starscream of Hell. I can see him engineering a conspiracy against Asmodeus that maybe never quite works out, like he's the leader of an opposition party that always makes a good showing, but never manages to clinch the vote.