The original Doom games were noteworthy for having big levels that contained lots and lots of enemies. I haven't played Doom 3, but I've heard that it has much more beautiful 3D graphics, and as a result you would be attacked by only a few monsters at a time (because too many would overwhelm the graphics adapters that were current when that game came out).
My favorite thing in the the original Doom games was getting the monsters to fight each other. If you could get an Imp so hit a Cacodemon with a fireball, for example, the two would get into a fight. Frequently I would lure some monster into the line of fire and as soon as it was hit, it would forget about me and go kill whatever monster hit it. This is more fun to me than just shooting everything. I hope the new game has this.
The specific rules: monster special attacks don't hurt other monsters of the exact same type... for example, Imp fireballs don't hurt Imps. But the zombie soldiers shoot bullets and bullets hurt anything, so you could get soldiers to fight each other. And anytime a monster hit a different kind of monster it would do damage.
P.S. Doom modified as a way to control processses on a system. Kill a process with a shotgun! https://www.cs.unm.edu/~dlchao/flake/doom/chi/chi.html
One side-effect of this is that processes on a system can get into a fight with each other. Two processes enter, one leaves. Not recommended for critical systems.