It doesn't mean a new back story for every event. What it does is allow us new settings as events emerge, based on where the discovery in Doom takes us and intertwining those into a more cohesive story. Wherever we do land will be evaluated for playability when it's time to cross that bridge.Der... So that means a new backstory for every event? I hope the current story with the discovery in Doom goes somewhere before then...
Honestly, I think you're better off keeping this event in Doom and just cycling the rewards. The majority of the other dungeons just aren't that interesting, even the revamped ones. Trying to crowd people into them will not work out as well as Doom did.