The true beauty of the DOOM engine is that over twenty years after its creation, people are still finding clever ways to bend and shape it into something amazing. Whether it is adding voxels or adding fatalities, people have done some crazy things with DOOM.
The craziest might be this strange little video, which shows off a mod that inserts a playable version of another DOOM engine game within a DOOM level. Shown playing both Wolfenstein and DOOM 1, this mod is more or less a low-fi version of the retro level in the new Wolf game, only infinitely more impressive.
If you want to try this out for yourself, there’s a forum thread started by the modder where they make the file available.