I just finished the "Doom 64: Merciless Edition" mod/hack which adds the missing "5th" difficulty to Doom 64 and various other features. Doom 1 and Doom 2 previously had a 5th difficulty called "Nightmare!" but this was cut in Doom 64. In the official remaster of Doom 64 which came out last year, this mode could be recreated by enabling fast monsters with the undocumented parameter "-fast". Inspired by this, I used the Doom 64 RE source code to recreate it with the "Merciless!" difficulty. Oh and if you want the other RE features but not the difficulty, the easier difficulties are still available. More details and ROM patches here:
http://www.romhacking.net/hacks/5634/And a video of it in action!
https://youtu.be/ZAxdGQfYdFc