D20 Dungeon Survival Game
Torchbearer is not d20, but based off of Burning Wheel (through the Mouse Guard RPG) and uses a d6 dice pool. It's a good game, and you might get some ideas from it, for sure.
The first thought I have upon reading your pitch, is what will drive the PCs to work together? Or is your game going to encourage PvP? Whether they're supposed to work against each other or together, either way could be fun. But the drive to play the way you expect the game to be played should exist.