Of course, some of the credit for the series' longevity has to go to the community that surrounds it. Players have created custom levels for its earliest entries (Doom 1 & 2), added new features, and even rebuilt the games entirely in the case of some specific mods.
Now, according to PC Gamer, Bethesda is using some of that community content to improve the aging classics for modern times. The company is accomplishing this by rolling out add-on support for the original Doom and its sequel.
Similar to some of Bethesda's community-created content features in other games (such as the controversial Creation Club), add-ons will be curated and approved by the company before becoming officially available. Like the Creation Club, this content will be accessible in-game when you boot up Doom 1 or 2's respective enhanced editions through the Bethesda launcher.
These add-ons will mostly be level packs and overhaul mods -- some new, some old -- but Bethesda has also independently added a few other modern features to the games. Some examples include support for 60 FPS gameplay (it's unclear if you're able to go higher or not), quicksave/quickload functionality, and even improved controls.
For a full list of the features arriving with the latest patch for Doom 1 & 2, check out Bethesda's official changelog right here. If you don't yet have the games, both titles will only run you $5 each on Bethesda.net.