Have one with a humongous rendition of Waterdeep to include the extensive catacombs and entrance to the vast underdark via the Yawning Portal Inn. Have an extensive Dock Ward littered with shady NPC's and danger lurking around every corner. The content and lore is endless. You could have underground player starting cities for the Drow, Gnome or Duergar races. Have an extensive thieve's guild out of Calimport. Have an extensive mage/wizard network out of Luskan. Have a barbarian race which starts out in the brutal tundra of the north. Quests that take you thru the Evermoors. Battle hordes of undead in Myth Drannor. Reclaim Mithril Hall for the Dwarves.
It wouldn't have to mimic any of the AD&D rule sets since they are hard to do in a computer game but at least use the lore and etc... I've been an RPG buff for decades and find the Forgotten Realms the most fascinating and imerssive by far.
Just my 2 cents. I've been dying for a good AD&D based game for a long time.
I realize Wizards of the Coats is so greedy that their royalties leave little room for profit which is why I suspect we haven't seen many titles in this century. (I heard a rumor at one point that Turbine's LOTRO was actually supposed to be AD&D based but Wizards of the Coast wanted to much)
Anyway, loving Skyim for the most part. (The interface is kinda frustrating but I can deal)