The first part of Doom Eternal's single-player DLC, The Ancient Gods Part One, is going to continue the story from the main campaign. You'll probably be better equipped to handle the tough demons and challenges the expansion will throw your way if you've finished Doom Eternal, but according to one of its co-directors, you won't even need to have purchased the main game to get in on The Ancient Gods.
Speaking with PCGamesN, id's Marty Stratton confirms that Doom Eternal's Ancient Gods DLC will be made available as a standalone purchase. "It's important as we want the DLC to reach as many people as possible," says Stratton. "In fact, you don't even need to own Doom Eternal to buy the DLC and play it."
That's an interesting approach to take with the expansion, especially considering that it'll build upon the story-dense events of Eternal, but perhaps it's reflective of the scale id promises for the DLC. Stratton says that The Ancient Gods "is every bit as grand as the main game was," likening it to a two-part film as opposed to a "made for TV version" of the original Eternal campaign.
If you're just not in the mood to play an entire Doom campaign or just want to skip ahead to the dragons and mechs, you won't have to wait that long to play The Ancient Gods Part One. Yesterday at Gamecom's Opening Night Live, id confirmed that the expansion will launch on October 20.
With free next-gen upgrades on the way for Doom Eternal and the next bit of DLC likely coming early next year, you'll have plenty of time and good reason to catch up with the full base campaign if The Ancient Gods ends up striking your fancy. If not, you'll only be out for the cost of the DLC and down a few hours of demon decimation for your trouble.