My (lame) idea of how RTC 3057 story should go:
After over one thousand years, there has been no more signs of the demonic hellspawn that almost resulted in mankind's destruction in the year 2050 [NOTE: may not exactly fit into the Doom timeframe/scene but hey].
The Marine who destroyed the apparent entity controlling the demons, "Baphomet" by some sources, swore that Hell was a wreck, that nothing would ever rise from it again. Still, the leaders of earth, along with the several million survivors who fled the planet were not convinced. As earth was slowly rebuilt, teleportation technology of any kind was permanently shelved and the military kept a close eye on the scene of the demonic invasion.
Centuries later, their guard was still high, but most of the scientists now said that it was unlikely that a future gateway from Hell could be opened from the other side. The threat of another demonic attack on earth was nil, or so they said.
They were wrong.
3057: Cybernetics have enhanced human's natural abilites for many hundreds of years. You are a cybernetically augmented Marine captain on the USS Opportunity, a scientific research vessel. Roughly two hours ago, an unmanned probe was sent into a nearby black hole, while the vessel remained a safe distance away from the hole.
What followed next you cannot remember. You awoke to sirens screaming, and emergency lights flashing. Daedalus, the ship's online computer system is blurting out distorted messages, something about a "foreign contaimination by unknown organisms". Screams and gunfire punctuate the messages. Strange animal noises can be heard in the background
You grab your sidearm and heard to the door. After all, what else would a Marine cyborg do?
While playing the partial conversion, you'd discover more about what happened, and maybe some unpleasant secrets about yourself. (this is my vision of what would happen later if my story was the official plot- which is obviously NEVER going to happen)
"Everything that deceives, also enchants"- Plato