Sigil is the unofficial fifth episode for the original Doom. It was helmed by none other than the game's co-creator John Romero and sported nine new story and deathmatch levels. As revealed in an interview with Bridgeburner, Sigil 2 is in the works.
It's likely that this will be a free game given that the first Sigil only required you to own a registered cop of the 1993 Doom. Sigil 2 looks like it'll be the same case but with 1994's Doom 2 as Romero claimed that Sigil's sequel will expand on Doom's sequel much as the first Sigil expanded on the first Doom.
As for its scope and size, Romero simply said that it will have "a lot of levels." The first had a total of 18 so perhaps something similar can be expected.
While it is an unofficial expansion, Sigil and its sequel are published by Romero Games and the last entry got its own physical releases. The limited edition was $166 while the standard edition was a standard $40.
There's currently no release date as Romero stated that he wants every map to have "really good consistency and progression" but he did tease what he wants to work on after Sigil 2's release.
Romero added that he wants to play around with another FPS hallmark - Quake. He said that it will "most likely be the next thing." Whether Nine Inch Nails will lend a hand again for the music is another question.
If you're wondering how Romero is publishing unofficial Doom games under his own studio with sold copies, the answer is fairly simple - Bethesda is okay with it and has worked with Romero in regards to Sigil.
It bundled Final Doom and the Sigil with Doom's console ports in 2019.