Spoiler:The idea of this is to replace the human enemies in Doom with humanoid demons. Because if you're like me, you've played levels set in Hell and thought to yourself "wait, why are there still possessed humans here? Did they like, follow me or something?" This would eliminate the problem and honestly make a pretty cool new enemy that would blend in nicely with the hell maps. I also imagine they'd be a bit beefier than their human counter parts. For example, the basic demon would have a multi-fire rifle like the SS officers and the machine gun zombies, and the chaingunner would have more health. Perfect as end-game replacements for the standard enemies.
The most efficient way I would think these monsters could be made is by just retexturing the existing zombie models but adding stuff like horns, etc.