If I understand you correctly, you want a DLC in which you can act out all the things that get talked about or planned in the current epilogue.
That is, as the Owl pointed out, quite a stretch because the current epilogue allows for a huge variety of player choices which would require a lot of work to be shown as content without necessarily having an overarching narrative that draws all of them together.
It'd be so much work that it'd most likely not satisfy anyone. I'd much rather Larian put that work into a toolset so we could create custom adventures instead.
So I think the epilogue as it currently is, is really nice because it provides closure but allows for a huge amount of variety. If there is to be a DLC, I'd assume it would be one in which the characters on ordinary power levels come together again to solve some larger problem - as Withers suggested - with Spawn Astarion and Karlach getting mini quests to fix their respective problems. But I think it's unlikely and I'd rather have a polished up Act3 with some Upper City chunks.
I don't really see how they could add the Upper City without something contrived. Larian stated the Upper City was always meant to be the climax of the game as opposed to a visitable area. Considering the overall goal in Act 3 is just to kill Gortash and Orin to trigger the endgame they'd need to come up with a plot contrivance to explain why the player suddenly needs to visit the Upper City and can't just fight the Netherbrain.
