I think what I was suggestion got lost somewhere along the way. Alot of the things I brainstormed above were just random examples and in no way a must have for any sort of DLC or epilogue. Mostly the thing I would like to see, is just roaming around Baldurs gate after the party and see it being rebuild. Being able to visit the characters you encountered along the way and see what they have been up to. For entire map creations, they could always just limit it to housing. For example: Visiting Gale, it could just be his tower in Waterdeep you can teleport to and visit. See him teach a class and catch up with Tara.
With Astarion, it would depend on his ending obviously, but you could visit the palace or find him in the underdark where the other spawn are perhaps. Or lurking in the sewers of Baldurs Gate as he said he took on a vigilante persona after all was said and done.
For Durge, I think it would be satisfying to be able to encounter the temple in all its ruinous glory one more time and have the option to just wreck it for good or hint at start restoring it by giving into your urges at last.
Id deffinetely like to see upper Baldurs Gate at the very least as an add on in a DLC, where a Selunite temple where Shadowheart is would be a neat way of wrapping that up.
The point Im trying to make is, just roaming around in the aftermath. No new character creation or massive questlines or storylines. I just want to be able to roam around the world I saved and encounter the characters and the endings they won. If that makes sense at all? Instead of just reloading camp or loading act three where nothing was done quite yet and progress is still something to be made.