I suppose we should have seen this coming.
Doom's modding community has been around for decades, recreating everything from Sailor Moon to the concept of piracy. Someday, you and everything you love will be replaced by a Doom mod. First, though, the new Doom's out, and dutiful modders are plucking out its insides and inserting them into Doom I and II's hollowed out torsos.
The most comprehensive effort so far is DBThanatos, Major Cooke, and Michaelis' DOOM (4) for DooM project. They're taking sprites of the new Doom's weapons (made by a modder named Neccronixis) and recreating their functionality in Doom I and II. Here's a video of what they have got so far:
Does anyone think some of these weapons look better as old-school sprites, or is it just me?
There's also The Doom 4 Emulation, which has a similar aim. New textures, sounds, music, and all that jazz, inspired by the new Doom.
Meanwhile, HorrorMovieGuy mocked up a version of new Doom's Glory Kill system in the game's ancient demon relic of a progenitor:
Lastly, released before new Doom even came out, Merser's Sprites and Enhancements adds Imps, Revenants, and Cacodemons from the new game to Brutal Doom, a gleefully gory mod that had a pretty profound influence on the new Doom. Just like that, we've come full circle. Or part of a circle? Wait, is this recursive? My head hurts.
I imagine these sorts of mods will only grow more elaborate and comprehensive with time. I hope one lets me fist bump action figures of myself.