This theory may sound weird, but hear me out.

[TheAlmightySunBro, Silkaire]

As we are all aware; DOOM THE DARK AGES is a prequel to both DOOM (2016) and DOOM Eternal. Depending on the ending of DOOM THE DARK AGES; it may be excluded from this theory. For now I’ll exclude it. SPOILERS AHEAD=
As we are all aware DOOM Eternal the Ancient Gods Part 2 ends with the Slayer getting into a coffin. In DOOM 2016 he wakes up in a coffin. Could this be connected? My only contradictions are the following: the coffin/sarcophagus is different, and the Slayer isn’t wearing his armor upon exiting the sarcophagus. (Could also be some lore I don’t know about) Now here’s the reason those might not contradict my theory: the coffin/sarcophagus could of been changed in design later down the line, and they could of always taken off the armor of the Slayer. My real theory here is “What if it’s all a loop?” It’s possible that when he went back in time and killed Davoth; maybe the world couldn’t run anymore because the creator was now dead so maybe the Sarcophagus is connected with the coffin and sealing the coffin caused a reset back to the reawakening of the Slayer. It’s possible. What do you think?

[UNBREAKABLE]

It's not a timeloop. The circumstances he ends up in the coffin are completely different both times.

Before 2016 (presumably at the end of Dark Ages) the hell Priests drop a temple on the Slayer and trap him in the box while he was knocked out. Then Davoth wrote the Slayer Testaments to warn his people not to open the box because the marine will just kill everyone. He remained there in hell until Hayden yoinked him out, then re-activated him at the start of 2016.

At the end of TAG the Slayer simply passed out, then the Seraphs and Father stow him away in the box again next to the other gods. Davoth is dead, Hayden became transfigured and the demons can't leave hell anymore. The Slayer has no reason to be active anymore since he achieved his ultimate goal.

[TheAlmightySunBro, Silkaire]

Originally posted by UNBREAKABLE:

It's not a timeloop. The circumstances he ends up in the coffin are completely different both times.

Before 2016 (presumably at the end of Dark Ages) the hell Priests drop a temple on the Slayer and trap him in the box while he was knocked out. Then Davoth wrote the Slayer Testaments to warn his people people not to open the box. He remained there in hell until Hayden yoinked him out.

At the end of TAG the Slayer simply passed out, then the Seraphs and Father stow him away in the box again next to the other gods. Davoth is dead, demons can't leave hell anymore and the Slayer has no reason to be active anymore.

Thanks for clarifying this. I still think my theory is interesting even though it couldn’t be canon.