I think beyond adding more enemies, the best way to increase the danger level is to increase enemy damage (rather than enemy hp, or other stats). This will ensure that the game is always dangerous, and force the player to be careful no matter how powered up Doomguy is.
Increasing enemy hp is simply powering down Doomguy's weapons and traits, relatively speaking, so the fight becomes similar to earlier ones on lower levels.
Increasing enemy speed can increase danger, but can lead to unfair situations, where an enemy could come in range of even a careful player, and before the player can act, fire two or more shots.
However, it could be fun to sprinkle a few "upgraded" monsters here and there, single monsters, unresurrectable, with upgraded stats in speed, firepower, and health.
Or even "Unique monsters" - bosses from Doom and other games.