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.
I don't think it would be very satisfying either. The current epilogues provide such a wide variety of plot-bunnies which, mixed with players' general tastes and fantasies, create such a broad assortment of possible futures, that they'd be hard to realise to satisfaction in a DLC. Player made content, be it in the form of gaming or as fan art or fiction is much better suited for this sort of thing. As cute as all the above ideas are.
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.
I don't quite believe them on that note. While not Upper City, the little garden behind the House of Grief looks very fleshed out for something that was never intended to be used (if compared to the other out of bounds areas) and the layout of Cazador's palace makes much more sense if you rotate the building 90° clockwise so the main entrance points to the Upper City Gate and the current weird entrance (which Astarion calls a spot to slip in) would be connected to the Chamberlain's office and the Spawn wing, which would make it a great side entrance to lure back "snacks". It's just a lot of little things. But overall a more fleshed out Act 3 would be lovely.
