Here's What We Know about the prospective sequel novel:
-The title '668 - The Neighbour of the Beast' was bandied about. I'm sure I remember Pratchett also suggesting '666 - The Number of the Bus', but can't find where I originally read either.
-It had a bunch of angels in it. We can probably safely assume that the new heaven/hell sequences from the show, and in particular the 'trial', were taken from sequel notes.
-There was a lot more of it than we'd been led to believe. My impression for, um, decades has been that they just sort of bandied ideas about, but Gneil indicates here that not only did they have a full plot, but they discussed it again much later, and Pterry considered it a plot worth filming. This also describes it as 'an entire novel plot'.
For the sequel series, we have this statement from Gneil:
"As Good Omens continues, we will be back in Soho, and all through time and space, solving a mystery which starts with one of the angels wandering through a Soho street market with no memory of who they might be, on their way to Aziraphale's bookshop.
(Although our story actually begins about five minutes before anyone had got around to saying “Let there be Light”.)"
And that's all we know.
Okay so. The 668 in the possible title suggests that Adam might still be hanging around? Good Omens the novel is very much Adam's story - Crowley and Aziraphale are in it surprisingly little, checking out for huge chunks of the action. The show changed this, but this plot predates it by a long way.
Given that technically the Antichrist has been born and come into his power, and Satan Himself has (briefly!) come to Earth, I'm wondering if the plot will deal with the Second Coming - y'know, the other part of the Book of Revelations. It has a certain symmetry to it: instead of "the Antichrist has gone missing", we'd have "the Son of God has returned, but he's gone wildly off-mission". If the 'trial' sequence from the show was originally the start of the sequel, that would leave A&C officially out of the loop - but a great potential source of officially unaligned interference if any angels needed a helping hand.
I have written this speculation several times now and keep getting tangled in it, so I'm just gonna stop. :)