Varied Enemies and AI
We hate to harp on it, but Doom 3 was really, really far away from the original theme: hell on earth. Through the pantheons of every world religion, hell is a place filled to the brim with demons and arch-demons of every shape and variety. When they cut loose on poor little Earth, we should get to fight more than two or three of them. Give us invisible demons, massive, hulking demons, demons made out of stone, tar, and sand—hell, give us demons we don’t even have words for. Different demons with different strengths and weaknesses will force us to adapt and think on our feet in ways that fleshy, anthropomorphic, more-like-aliens-than-demons demons never did.