So, John Romero, one of the creators of the original Doom games, is releasing an unofficial fifth edisode, named Sigil, for the original Doom. It'll be released as a free mod in February, 2019, as well as limited physical editions with some fairly cool stuff.
Here's some more material if you want to learn more:
- The official site for Sigil
- The announcement trailer
- Rock, Paper, Shotgun's news article on Sigil
- John Romero's gameplay stream video