If you are still looking at this thread you created, I would have to agree with the person who recommended the Elderscrolls games, particularly Morrowind. With the Construction Set, you can do almost ANYTHING, including creating those dungeons you mentioned. You can create whole new landmasses and villages and much more, if you want. Plus, like BG2, it has tons and tons of excellent mods. Clothing mods, race mods, quest mods, NPC mods, creature mods, land mass mods, armor and weapon mods, dungeon mods, spell and potion mods and so on and so forth.
The game itself has a LOT of interaction and there are mods that add wonderful joinable NPCs, some of them romanceable, if you like that kind of thing (which I sure do). If you do get Morrowind (get the Game of the Year edition, as it comes with both expansion packs), I highly recommend you get the Better Bodies mod and a good NPC head replacement pack as well as some head packs for your own PCs. You can find those mods and more at Planet Elderscrolls. Without Better Bodies and a replacement headpack, the characters will look like He-Man and She-Ra segmented action figures, but with the mods I recommend, they become very nice looking indeed. You won't believe how many great mods there are for that game.
I find the storyline very engrossing, however, you can ignore the main storyline if you want. I spent months playing Morrowind without touching the main story and had blast. For me, Morrowind and the BG Series tie for best RPG.
As for Neverwinter Nights, I hated it at first, but came to enjoy it thanks to both the expansion packs and the player created modules available at Neverwinter Vault. For me, Hordes of the Underdark made the whole game worth it. THAT has the interaction (and romance) that we love from BGII. And many of the modules are brilliant.