It should still give the player choices. It seems the only two who, with the current endings, aren't up for a new adventure are Lae'zel and Shadowheart, the rest are all up for more as a group. Astarion and Karlach are just tied by circumstance, but a 'fix' for Karlach is hopefully being worked on already.
By and large, I think with an expansion, we wouldn't necessarily need our current companions. They could largely just appear for an occasional extra scene or personal quest.
That being said, I think Avernus is a good choice because there's a ton you could do with it.
-BG falls into it so most of the companions / Tav end up there by proxy
-Tav could be going to hell, either alone or with their lover to look for a cure for Shadowheart's hand, Astarion's vampire curse, etc.
-Tav is there because they went with Karlach
-Tav died or became a mindflayer and their soul ended up in Avernus.
-Githyanki Tav is in hell for some Tiamat mcguffin to help in the fight for or against Orpheus.
Even most companions who died can be connected back to it cause souls can always end up there (e.g. Karlach's soul might still belong to Zariel if she died). That being said, I think Wyll and Karlach would be the ones taking center stage of the returning companions since their stories feel the most incomplete. We could also get some returning villain, especially ones who didn't quite have as much impact on the story as they should. Cazador specifically comes to mind, with him having made a contigency pact with Mephistopheles (which could also tie in vampire spawn Tav via a kidnapping / soul exchange sub-plot).
I guess what I'm getting at is that there's a lot of room to tie everything back together in a way that feels satisfying.
