GoS isn't a splatbook but it still has a lot more additional rules in it than the typical Adventure book. And it may be the case that Wizard's is testing putting more rules along with Adventure Books, to push the "rules buyers" into buying Adventure books too. They're really pushing how The Descent has rules for warmachines and contracts.
Still trying to figure out what product these could be for... I doubt it's another Xanathar's, for the reasons you and others have brought up. It also seems like too much for an Adventure book. Is it possible Wizard's is moving to a new Setting Book every year, and that this is for something like Zendikar/Dominaria/Theros/Greyhawk?
I also really like that "Rabiah Scale" article you posted, it's got Dominaria, Ravnica, Zendikar, Innistrad and Theros as in their top "most likely to make a new magic set for."
If the same scale works for "new D&D setting," I'd rank them least to most likely Innistrad, Dominaria, Theros and Zendikar.
Innistrad is unlikely to me as it's mostly "more Ravenloft." It's cool but it wouldn't be that unique.
Dominaria is popular but I put it as less than Theros and Zendikar because those two settings are coincidentally getting new Card sets next year.
Not sure if Theros or Zendikar are more likely for a setting book... neither has a Planeshift, and I believe Zendikar is the more fleshed out but I'm not very knowledgeable on it.