The upcoming port release of Doom 64, which is scheduled to be released on March 20th, 2020, alongside the long-awaited Doom Eternal, will feature a brand-new chapter as a continuation of the original Doom 64 story, and a direct follow-up after Doomguy defeats the final boss of the game.
The new chapter also promises to add in a "bit of lore" that seems to cater to fans of both the original and the modern Doom series.
“On our end, persistent players will have the opportunity to unlock a new chapter in the Doomguy’s saga, taking place shortly after [Doom 64’s] original campaign concludes. The Mother Demon you defeated in that outing had a sister, and since you’ve been messing up Hell non-stop, she tries to get rid of you by sending you away. If you can make your way back and take revenge, you’ll be rewarded with a bit of lore that fans of both series, new and classic, should enjoy.”