So, what would your ideal storyline look like, without reference to the rules? Are you thinking survival horror, with lots of stealth? PCs trapped somewhere and they try to find the keys to get out alive while avoiding the monsters, that sort of thing?
Give us an idea of what you'd like the game to look like, and maybe we can offer some suggestions on tweaking D&D to do it.
D&D does heroic fantasy well,] but I do think it is more versatile than a lot of people give it credit for being. Last GenCon, I played a straight-up, no-combat heist scenario in D&D, and we all had a blast.