I take it that everyone's aware of the Skyrim easter egg in Doom 4/Doom 2016, right?
Well, I've been thinking for some time of the possibility that the "Hell" seen in the Doom series is a plane (realm) of Oblivion from The Elder Scrolls, that was connected by accident to our dimension, and that the various demons that Doomguy fights are actually Daedra.
There are many realms of Oblivion that have been both seen and unseen throughout the TES games. In the fourth game, The Elder Scrolls IV: Oblivion, we've seen at least three realms that could easily be mistakable for the traditional fire-and-brimstone portrayal of Hell: the Deadlands of Mehrunes Dagon, Peryite's Pits, and the realm that Boethia/Boethiah sends the player to for his shrine quest. Hell could be one of those three, or even yet another realm that has this general appearance.
I'm aware that some people might roll their eyes at the idea of TES and Doom sharing the same universe, and think it might ruin the mythos for either series. But it can work if it's done right.
Both series prominently feature interdimensional travel, and we've seen that the physics and laws of dimensions can change radically in both series. One dimension could have actual stars and planets, but another might not. And our dimension could run parallel to Oblivion and Mundus (the dimension where the world of Nirn, and thus the continent of Tamriel, is located). There are many ways of traveling between a realm of Oblivion and another dimension, so establishing that Hell is a plane of Oblivion is all that would be necessary to establish some sort of connection between the two franchises.
And in case some people are concerned that copyrights would get in the way, here's a reminder:
Doom belongs to id Software.
The Elder Scrolls belongs to Bethesda Softworks.
Both companies are owned by Zenimax. Plus, Bethesda has already been involved with the Doom series. So chances of a legal hassle are positively minimal.