Gonna put a couple more ideas in here. Someone's still reading them right?
Gib deaths:
If you die with -10 or less HP, you let out the higher pitched scream as opposed to the normal one.
If you manage to do twice the damage normally required to kill an enemy (eg kill a human using 20 damage, when 10 is enough) then their death sound is replaced with DooM 2's gib sound (DSSLOP), and they leave a pile of gibs instead of a corpse. This could maybe stop Arch-Viles from ressurecting them.
Nightmare:
Too many people are beating nightmare now. Add in the other special feature DooM 2's nightmare had: Reduced fire delay. Make all enemies fire with half or maybe even a third of the normal delay, but reload and move at normal speed.
If this is included, you might want to look in to the late game ranks, because nightmare would become almost impossible to beat and everyone would be a general forever.