I mean if we're going full imagination here with a quest in Avernus, I'd also love to have Zariel herself involved. Perhaps not like, as a boss ('cause dismantling an archdevil of a whole layer of hell surely calls for a bigger separate questline, it's not Raphael), but to make an appearance. To have Karlach show her the metaphorical (or literal) middle finger like, say, Lae'zel does with her Queen. I know Gortash takes the role of a former abuser in her questline for now, but if we're talking, like, comparisons with other companions, then it really feels like Zariel should be her ultimate big bad, while Avernus would be her dedicated location (a la creshe or Shar's Gauntlet or Cazador's mansion, etc)

I also don't think Larian would add THAT much content here, but it's fun to imagine!

[Linked Image from tarraxahum.neocities.org]