The best thing I can come up with is something akin to Dragon Age : Origins Awakening DLC. Some companions from the base game show up, but not all of them. Some of the companions you get to use in it are completely new and only locked to it. If you don�t have them because they died or something else in the playthrough, then another character is out there as a fail-safe. Because an Avernus DLC really seems to most plausible as a post-epilogue DLC (to me at least) if they go that route, I don�t think it makes sense for every companion to really be there. I can see obvious justification for Karlach and Wyll, and depending on what story they want to tell, I could see honestly Gale being there, since he�s always trying to learn new stuff about magic. Everyone else is kind of a hard guess to make though, but if anyone could do it, I would put my faith in Larian.