My idea: THE DUNGEON ITSELF IS ALIVE AND HOSTILE.

Think about it, Illithids are so adept at warping body they made the brain golem. Who's to say there isn't' some brainless monster that occasionally flails at the PC's with an odd tendril or the floor opens up into a fanged maw that eats you alive.

For random mooks I'd say undead corpses made from other adventurers who failed. Stress how they have oddly useful if mostly broken equipment, as if they went in to explore the place themselves. For added effect if a PC falls animate him as a zombie