I think between the epilogue scenes with her, withers message, larian statements about dlc and expansions, and the now deleted tweet from a larian dev I think an avernus dlc is a matter of when not if. In that case it would be asinine to go all the way to avernus and NOT go with karlach to that forge to give her a working heart she can go back to baldyrs gate with. I can't see larian doing that since she is like you said the only origin companion who doesnt get the one thing she wants. That being to just be normal again and not worry about devils, gods, demons, or wars.