Like how Doom 64 was darker, creepier, slower, and more atmospheric than the classic Doom games? Then use the D64ifier to D64ify your game. Make Doom, Doom II, Final Doom, No Rest for the Living, and most other vanilla mapsets look, sound, and feel like Doom 64.
Preview of the improvements being worked on for the next release of the Hi-Res Texture Pack.