I just finished watching a theory by beloved theorist MatPat called Game Theory: The Fortnite x Kingdom Hearts Crossover | How Doom Unites All Games! which is a fine enough video which you can find here. I did enjoy the video as his videos are entertaining but he does make a few leeps and go fast and loose with the words "canon"
So I thought I'd go over and come up with my own theory. He starts with the connection that William J Blazkowicz from Wolfenstein is the grandfather of Commaned Keen a.k.a William Joseph Blazkowicz II who is the father of Doomguy B.J. Blazkowicz III and that's all established via Doom RPG and
this tweet by one of the original creators.
We can add Quake in there due to the rabbit connection in Quake Champions which they also point out. The character in Quake Champions is clearly the same Doomguy. Altho Quake seems to take place outside of traditional universes so might even be just a verse in and of itself.