Brutal Doom and various other mods are already fantastic! Now we're getting mods for Doom 1 and 2 that add many of the new weapons, features and enemies in sprite form into the original Doom engines.
The DOOM (4) for DooM project is stuffing all of the new Doom into old Doom and it looks pretty damn awesome. The video above highlights how the project is coming along so far. It's only the famous 4 Mancubus fight that's ceremoniously featured in the new Doom as a nod to the old Doom but this is definitely high on my wanted list when it's finally finished.