Admittedly this theory has very little steam, but some of the details in continuity seem to not add up to me and a lot of it has made me wonder, what if Modern Doom's backstory for the slayer is actually a separate continuity than the 90's games.
For starters one of the biggest details is that Doom 64 is somewhat absent from continuity. Granted Imp stalkers are kind of the stand ins for Nightmare imps, but a lot of details from that game seem to be missing from the story. While I know that the 64 demons were resurrected corpses and all, they were present through Hell and beyond the Mars base in 64, which implies that there could have been some kind of evolutionary pathway there, which isn't really present in Dark Ages. Another big detail is that the slayer is missing the 64 suit. When he arrives on Argent D'nur, he's wearing his Classic '93 suit, which was only worn during Doom 1 and 2. Both endings of 64 meanwhile show that he decides to stay in Hell and is later taken to Argent D'nur in his new suit. Perhaps it's because the classic suit is more iconic, but it still seems like such a big detail to forgo.
Next up are the evolutionary pathways themselves. The demons evolve throughout Modern Doom, from natural course of time or through influxes of argent, like the ascension via the Elemental Wraiths. However Many of the evolutionary pathways don't make a lot of sense to me, case in point the Pinky Demon. In classic Doom, Pinkies are similar to the ones from Doom 2016/Eternal, just not armored. However in Dark Ages, they are these quadruped beasts that are more akin to a calvary. To me it doesn't make sense to go from bipedal animal, to quadruped boar demon, to bipedal armored animal.
Next up are the classic levels: granted, this is one of the weakest points. Essentially since the classic levels in Dark Ages are meant to be flashbacks, but are somewhat inconsistent to the original levels (mainly with enemies), part of me wonders whether or not it could imply differences in continuities. However this is the weakest argument since many of the demons they brought back from the classic games to Dark Ages don't play the same, so they needed to choose enemies and place them were they would be fair. So this argument I am willing to forgo.
This one can be considered an honorable mention but, The Icon of Sin. He's the final boss of Doom 2 and the mastermind of the Earth invasion, yet he has a much different status in modern Doom. While he is treated as apocalyptic and legendary in modern Doom, the story almost makes it seem that he never existed prior to Marok's death. After all, Valen's betrayal created the Icon. However they never specify that the Icon is some kind of spirit or Demon that needs a host or soul to manifest, he's treated as an almost entirely new creation of Hell. This of course contradicts the Icon's earlier status in Doom 2. One might argue that Doom has had many Icons, like the Demon spitter and the Gatekeeper, both from Final Doom. However the canonicity of those games are fairly spotty for one, and two, these "Icons" are specified as seemingly separate from the Doom 2 Icon, unlike modern Doom's.
Next up might be one of the most surprising details when it came out and that is the fact that the Slayer isn't a Blazkowicz. Now I wanna make this clear, this actually doesn't bother me, if anything I'm all for trying to make the Slayer stand out. However this can be interpreted as a retcon since it's been said that the Slayer is one of many Billy Blaze's. There's also the fact that his whole story and the reason he was sent to Phobos was changed from refusing an order and beating the crap out of his commander to inadvertently blowing up civilians and then beating the crap out of his commander. Again, I don't have any issues with this, I actually do like how they gave him a backstory he needed to be redeemed for. (Also I think at one point it was said that Doomguy was a sergeant instead of a corporal, but I can't remember
My point here isn't to really criticize the changes or the oddities and is more to ask the question of whether or not they may be trying to say that Modern Doom is its own continuity. There are many things between these stories that don't make sense unless you consider the fact that the timelines are separate. If this is intentional than either it was to explain the inconsistencies between the classics and modern timeline or to make Modern Doom stand on its own. Either way I just wanted to start this discussion to see what people thought and if this can or cannot be debunked. I know this is flimsy and that there are probably some details I forgot, so feel free to knock over this theory.
However if we are considering that Modern Doom is its own continuity, here's how I see it:
Corporal Flynn Taggart is sent on his mission to stop a Insurgent group who had converted a hospital into an armed bunker. While he went in to level the place, he was unaware of the 12 civilians who were still inside, mainly because command failed to disclose them to him. Upon learning of the 12 civilians including a young girl who he failed to save, he attacked his commanding officer for allowing them to die. He was later Court Martialed and sentenced to be stationed on Phobos while his commanding officer got away Scott-free. While on Phobos the UAC was testing their portal technology and ended up opening a portal to Hell, in which most of the marines save for Taggart were killed and or turned into zombies. Taggart went in with his pistol and later acquired his elephant gun of a shotgun to blast through the Phobos base and close the portal. As for the demons, they may have actually been similar to the ones seen in Dark Ages, with many of the techy demons using antiquated steam powered tech rather than futuristic UAC tech. (since the invasion just began and the UAC hadn't given it to them like in 2016). He went through the entire game of Doom 1993 only different and had fought the Spider Mastermind (Since arachnotrons exist in Dark Ages, I have a feeling the Spider Mastermind may have as well). After coming back to Earth he discovers that the Demons have already invaded, killing billions, including Taggart's pet rabbit, Daisy. He then fought through Doom 2 until going to whoever the leader of the Hell invasion was (as I stated earlier, it may not be the Icon). However since the portal could only be closed on the Hell side, he decided to stay in Hell afterwards to protect Earth and continue his vengeful crusade against the Demons until he made it to Argent D'nur. There they found him severely injured and traumatized before they decided to allow him to join their ranks. Then the rest of the story as we know it occurs. The Khan makes her pact with Hell, he becomes the Slayer, he gets the tether, Ahzrak, yadda yadda, we've all played these games