I'm basing this off my prediction in the thread about the impending announcement. I'm not sure if this is exactly what I want, but I think it makes sense, and I'd be very happy if they follow this route.
Adventure book: The PCs find a spelljammer in the Realms somewhere, possibly Lantan. The adventure is made up of loosely connected episodes of locations beyond Toril that the PCs visit while on the spelljammer. How about this premise: They explore the ship and it "turns on," and starts going to random locations outside of Toril. It could even be a kind of random thing, like the Deck of Many Things, but the "Panel of Many Places," with the PCs pressing a button on the spelljammer's dashboard, and off they go, sailing the Astral Sea. Perhaps the spelljammer is sentient and trying to remember what it is, and visiting places it had been to previously. The PCs gradually put together the mystery, and learn how to pilot it over the course of the campaign. The final destination is none other than...the city of Sigil. Which leads me to the second book:
Shemeska's Manual of the Multiverse: This is a combination of Manual of the Planes and Planescape, with flavors of Spelljammer and Magic's planeswalking. It includes a gazetteer of Sigil and the Outlands, and an overview of the planes, along with rules for different approaches to exploring the multiverse--the doors of Sigil, spelljamming on the Astral Sea, and planeswalking. It would also discuss different cosmologies, emphasizing that there is no correct, singular one, just a wide variety of lenses to view and explore the multiverse from.
This opens up the door to as many planar adventures as WotC wants to publish, and the connective paradigmatic tissue for other worlds.