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.
Slowly working my way through the Hi-Res Texture Pack to improve everything. Re-created BigDoor3 from scratch first, and then blended it with a smoothed out version of the original texture to try to get as faithful of an appearance as possible while still being an improvement. Also creating new normal maps and specular maps.