It would be a hell (heh) of an adventure to go down to the Hells. I hear Larian does not really do DLCs but I truly hope they do one for Avernus/the Nine Hells. There are already plenty of connections: Raphael is the son of Mephistopheles who is the ruler of one of the layers, there is Mizora who works for Zariel but some speculate could be related to Levistus (another archdevil of another layer), and Zariel herself has an extremely interesting story, and Karlach's own story could fit in there pretty neatly if one thinks hard enough.
As for where the House of Hope is, I've been trying to find a video or in my own game where Karlach says it is in Avernus but so far I only found her saying "in the Hells", which could mean any of the Nine.
