Freindly fire. Self explanitory.
Drop mags: anything inside the magazine is lost when reloaded. single loaders like revolvers and shotguns go uneffected. perhaps hide the UI on the ammo left in the mag.
a combination mode of the two above. [ I admit it sounds painful but I actually enjoy playing with such limitations.]
gorilla warfare: tripwires on some doors, booby trapped chests and tinkered barrols that activate when someone gets in proximity, lighting the fuse.
dark visions: taking insperation from the mage tower or tower of treachery, feeding players odd visions, maybe images based on their character choices, deafening whispers, false terrain that cuases players to split.