Here's my shitty theory I have been sharing for a while:
Original timeline: (First Generation 1993-2004)
Ultimate Doom->Doom2->Final Doom (two split timelines or both canon)->Doom64 = Original Doom Marine, You.
Second timeline: (Second Generation 2004-2016)
Doom3->Doom2->Final Doom (two split timelines or both canon)->Doom64 = Original Doom Marine, You.
Other Marines in Doom3-> Resurrection/Lost Missions= Bravo Team Guy >Resurrection of Evil= Marine Combat Engineering Guy. Two years later.
Second alternate timeline: (Second Generation 2004-2016)
Doom RPG->Doom2->Final Doom (two split timelines or both canon)->Doom64 = Original Doom Marine, You.
Other Protagonists in this timeline: Doom2 RPG = Stan Blazkowicz and friends. One year later.
Third timeline: (Third Generation 2016-Present)
First timeline->Doom2016->Doom Eternal
Second timeline->Doom2016->Doom Eternal
Second Alternate timeline->Doom2016->Doom Eternal
You can find many references to these timelines in Doom 2016.
Doom 2016 References:
To me, Doomguy (Classic), Doom Marine(Doom 3) and Doom Slayer(2016-Eternal) are the one and the same. I hope Id makes an interesting story in Doom Eternal about him as they have been working on that as the game suggests and not make him OG Doomguy just for simply fan-service. I don't need Daisy's pics or her cell or any other silly reference Doom Eternal is hinting to know He is the OG Doom Protagonist will all know and love. Guess, what about if he is not OG Doomguy you want?? I'm sure you will be very disappointed and all your dreams will be broken and those who said: "He is not Doomguy" would have won...
But what I see Doom Eternal won't let that to happen but I think it is not necessary to include hints if he is OG Doomguy or not. He is the Doomguy for me..
When I played Doom 3 and reached the final level, I realize that Doomguy was seeing a coffing with a Doomguy buried there. Even the Martian hero was Doomguy...