Yeah, the Doomguy is a descendant of B.J.
Also yeah, in DOOM2016, you found these fragments of, I think it was the bible of hell or something, called the Slayer Testament, which described the Doom Slayer as leader of the Night Sentinels, Guardians of Arjent D'Nur, who began his campaign of revenge against hell after the demons took over his world, it's mentioned that this was during the "first age", and according to some of the data you can find, the demons are currently living in... the fifth age I believe. Don't know how long each age is but it is mentioned that studies of the Praetor suit identify it as being old as hell (heh ), and the Slayer had been killing demons well before receiving it. There's also that whole thing about the Slayer being powered up by the Seraphim, who bestowed upon him strength and speed, then the thing with the Titan, hell's last hope which was crushed by the Slayer, etc. There's a bunch of stuff there that pretty much states the Doom Slayer is not even human.
In fact presumably it was thanks to him that the demons were able to do all that as it is mentioned that the demons promised the return of his son in exchange for access to the Wraiths, however, instead of his son, they returned to him the Icon Of Sin, and proceeded to take over/kill everyone in Arjent D'Nur. Afterwards he began his crusade, killing the crap out of any demons that were in his way.