Angel of Bad Aim. (or high sensitivity, keeping to the shooter roots) you can only aim no more than 5 squares around yourself. If you want to shoot enemies futher away, you have to approximate the shot.
Angel of Explosions. (carnage's taken). All explosions are double (or 1.5 if double's too much) sized. Both enemy fire and your weapons, and barrels too. You could use this to your advantage with larger barrel explosions and rocket blasts, and the BFG taking out most of a level in a single shot, but even imp fireballs would now have sizeable explosions.
Angel of Arenas. All random levels not hells mazes have all walls removed on entry. Might be impossible.
Angel of Buckshot. You start with a basic shotgun instead of a pistol, and can only use all kinds of shotguns.
Angel of Taveling light. Inventory holds only 10 items. Probably a more annoying than fun challenge.
Angel of Ammo. All ammo packs contain 1/3, rounded down, normal. Possibly would make ammo restricted by more than inventory space.
Angel of Bloodthirst. Every 10 turns you go without killing anything, you lose 3 hp.