I already posted this on the Doom Wiki, but since the Fallout series is also involved in my theory, I thought I'd share it here as well:
For those unaware, the "Zeniverse" is a hypothetical shared universe that contains the various series owned by ZeniMax Media and the companies they either own (Bethesda and id) or are associated with to some degree (Interplay, Raven Software, Obsidian Entertainment, Black Isle Studios, etc.). This includes Rage, Doom, Wolfenstein, Fallout, and The Elder Scrolls (among others). Since for a number of reasons, there's no way that all these series can possibly follow a linear timeline, I'm taking a page out of Nintendo's book by proposing that alternate timelines are involved.
Timeline one: Classic Era (BJ kills Hitler)
Spear of Destiny-->Nocturnal Missions-->Wolfenstein 3D-->Commander Keen-->Fallout: Brotherhood of Steel-->Fallout Tactics-->Doom (1993)-->Doom II: Hell on Earth-->Final Doom-->Doom 64-->Quake
Timeline two: The New Order Era (Nazis win WWII)
Spear of Destiny-->Nocturnal Missions-->Return to Castle Wolfenstein-->Wolfenstein (2009)-->Ultimate Challenge-->Wolfenstein: The Old Blood-->Wolfenstein: The New Order-->Quake II-->Rage-->Doom 3/The Lost Mission-->Resurrection of Evil-->Enemy Territory: Quake Wars-->Quake 4
Timeline three: Wasteland Era (Hitler kills himself)
Spear of Destiny-->Nocturnal Missions-->Return to Danger-->Doom (2016)-->Fallout-->Fallout 2-->Fallout 3-->Fallout: New Vegas-->Fallout 4
So as you can tell, the three timelines at first start out identically, but change after B.J. Blazkowicz defeats General Fettgesicht at the end of Wolfenstein 3D's Nocturnal Missions. In one, B.J. is captured by a vengeful Hans Grosse and brought to Castle Wolfenstein. In another, he is captured while attempting to raid Castle Wolfenstein. And then there's the third, where the Spear of Destiny is recaptured by the Nazis and B.J. has to recover it again.
In timeline one, Hitler is killed by B.J. and World War II ends earlier. In timeline two, the Nazis win the war, thanks to knowledge of future technology given to them by the mysterious demon Abigor. In timeline three, World War II ends just as it does it in real-life: Hitler commits suicide, and the Allies drop atomic bombs on Nagasaki and Hiroshima.
In timelines one and three, technology across the world is improved, until resources begin to dwindle and a nuclear war follows in 2077. Though similar, the aftermath of the war in these two timelines differ significantly. In timeline two, the Nazi government is eventually abolished, but mankind is nearly wiped-out by an asteroid that collides with Earth in 2029.
Union Aerospace Corporation
There is one constant in all three timelines, regardless of how World War II ended: The Union Aerospace Corporation (UAC). Even in spite of an apocalypse in each timeline, they have managed to build their own bases and maintain a population on other planets. In the first timeline, they have bases on not only Earth and Mars, but other planets and moons, including Mars' moons Phobos and Deimos, and Jupiter's moon Io. The UAC also has a presence in the second and third timelines, but they don't seem to quite as vast by the time they make contact with Hell: The only known bases appear to be on Earth and Mars.
Since the UAC hasn't been mentioned (yet) in any of the Fallout games (which all take place in the United States), I'm assuming that either UAC was a secret organization, or they originated from Europe, Canada, Australia, or another part of the world that would prominently feature English-speaking people.
Since there's no word on how the Great War affected Australia, it's highly possible that they were isolated enough to not only escape a direct attack from either the United States or China and maintain a population that could expand themselves space-wards.
Canada's another possibility, since the United States annexed Canada before the Great War, presumably for their resources. Perhaps this would also allow American influence to spread to certain Canadian organizations, which would explain why Vault-Tec's logo appears in the 2016 Doom reboot. Another hint that UAC might be connected to Canada is that the settlement of Ronto (which has been speculated to be the post-War version of the Canadian city Toronto) is said to have a military force that rivals that of the Pitt. In the Doom series, the UAC is known for its accomplishments in the development of military-grade firepower, and it's reasonable to assume that UAC's weapons have been sold to its citizens in whatever country they came from.
There's also a picture of a Canadian citizen being confronted by American soldiers in the first Fallout game's intro. The Canadian wears combat armor similar to that worn not only throughout the Fallout series, but also by UAC marines in the Doom series.
Two Fallout timelines?
Yes, I suppose many of you are curious about why Fallout: Brotherhood of Steel and Fallout Tactics are both in timeline one, yet the other Fallout games are in timeline three. Well, it's actually pretty simple: Brotherhood of Steel and Tactics have content that contradict the other Fallout games, and aren't considered canon by Bethesda because of this.
I suppose you're wondering "But Brotherhood of Steel references the events of the original Fallout! Why isn't that game included in timeline one?". Because the original Fallout doesn't really happen in timeline one. I suspect that extraordinarily similar events would happen involving the Vault Dweller and the Master, but the events themselves would have played out differently than in timeline three. How so?
Well, for example: Attis, the super mutant leader from Brotherhood of Steel, seems remarkably similar in a number of ways to the Lieutenant (similar design/personality, same voice actor, etc.). I assume this is because Attis and the Lieutenant are one and the same. In timeline one, the Lieutenant survives the Master's defeat instead of being killed by the Vault Dweller (which happens in timeline three), and moves to Texas to create a new super mutant army.
Mind you, I'm uncertain whether cancelled titles like Van Buren and Fallout Extreme would fit into timeline one. Although I suppose they would have to, since Fallout 3 and Fallout: New Vegas would contradict Van Buren's content.
The Elder Scrolls
Now where does The Elder Scrolls fit into this whole thing? After all, the laws and physics of The Elder Scrolls drastically differ from that which we've seen in Wolfenstein, Fallout, Rage, and certain parts of Doom. Most people have probably read the Kotaku article that speculates that The Elder Scrolls is set in the same dimension as Fallout, and that the former is either the past or the future of the Fallout series. Needless to say, this theory is--to put it nicely--lavishly ludicrous.
No, my theory is this: The Elder Scrolls is set in a different dimension altogether from Fallout, Wolfenstein, Rage, and the Earth/Mars side of Doom. In The Elder Scrolls, there are an infinite number of planes in Oblivion, one of the realms that form the TES universe. The planes of Oblivion are inhabited by a number of supernatural, interdimensional beings known as Daedra. Daedra are commonly referred to in The Elder Scrolls as "demons". There are many types of Daedra, including ones that would be a dead-ringer for the traditional depiction of a Christian demon.
And by coincidence of coincidences, Doom is all about humans from Earth fighting interdimensional beings that are nicknamed "demons" by the humans of that dimension, due to their appearance as well as their connections to the occult and supernatural. Could it be that the "demons" in Doom are simply a menagerie of obscure Daedra from a lesser-known plane of Oblivion? A plane of Oblivion that was connected by accident to the Earth/Mars dimension when the UAC was performing their experiments with the teleporter?
Of course, this would be the UAC of timeline one, where Doomguy originated. After he defeated the Mother Demon at the end of Doom 64, he remained in Hell to make sure no other demons would try invading his dimension. Time passes, and Doomguy continues to battle demons throughout Hell. His battles take him across time and space. Eventually, he is imprisoned by the demons, until he is discovered by members of a UAC team. But this is the UAC of timeline three, rather than of timeline one. Either Doomguy entered an alternate timeline at some point, or the timelines work under "multiverse theory".
Here's a video that could better summarize things better than me: https://www.youtube.com/watch?v=vdKn2-YzzxI
Going with this video's theory, this could mean that the demon Abigor from Ultimate Challenge is an underling of one of the Daedric Princes (likely Hermaeus Mora, whose sphere includes forbidden knowledge), or perhaps even a Daedric Prince himself. He may even be the mysterious "Demonic Voice" heard prominently throughout the 2016 Doom.
In Wolfenstein: The New Order, there is an ancient order known as the "Da'At Yichud", which has harnessed advanced technology that seems alien compared to the rest of the world. Not only that, but their tech appears to resemble that of the Dwemer (Dwarves) from The Elder Scrolls. In the third game, Morrowind, it's been established that the Dwemer had disappeared long ago, except for one: Yagrum Bagarn, who was Master Crafter of the Dwemer from the First Era. Lore states that Yagrum didn't disappear like the others because he was away in another dimension, though it's never specified where this dimension was, nor what he did while he was there. Perhaps Yagrum visited the Earth/Mars dimension thousands of years ago, met the people of Da'At Yichud, and helped them build all their wonderous tools and inventions before he returned to Nirn.
Also, since The Elder Scrolls Online tends to have a number of huge differences that deviate it from The Elder Scrolls lore, I propose that it, along with the mobile version of TES IV: Oblivion, occurs in a different timeline from the main installments. It is likely that it would contain the version of Hell, as well as other dimensions encountered in the "New Order Era" timeline, while the "Classic Era" and "Wasteland Era" timelines would intersect with the main TES series and pre-Online spin-offs (Battlespire, Redguard, and the Travels games).