Take the movement and combat system from Dark Souls.  Create four to six clearly distinct and powerful classes using their pre-existing abilities, stats, and items.  Set up multi-player code to allow 2 to six people to play together in co-op mode. Create some dungeon-like maps.  Populate those maps with bad guys using the procedural bad-guy spawn controlling algorithims from Left for Dead 2.

Just sit back, and imagine the awesomeness.  Hacking through orcs and skeletons and zombies and whatnot, Left for Dead style, using the dense combat mechanics of Dark Souls and the combined-arms awesomeness that you always imagined was hiding deep within Gauntlet.

Oh yeah.