The thing about the rules in the setting books so far is that-- while they could certainly be used generally-- there was a strong argument for how they were tied to the particular setting (with maybe the exception of the Bladesinger?). It's why I wouldn't be surprised to find psionics rules in a Dark Sun book. Sure, a lot of people will want them outside Dark Sun, but you kind of get why it would be put in that book. WotC have tried to focus heavily on story for both their adventure and setting books, so it seems like rules elements need to have a strong tie-in to the story.
I have a hard time seeing that working with the 'alternative class features'. They are pretty general and don't seem to have any link to a particular setting. Thematically, they seem a better fit for a Xanathar's type book-- a place to expand rules without a strong tie to a given adventure or setting. I'll admit the subclasses we've seen in the prior UA's do seem to hint at some interestingly flavored setting. But maybe the class feature stuff is separate?
Main argument against a Xanathar book is that they might not want to do one for another year. But who knows? The other possibility for the class feature stuff being in a 'setting' book would-- as others have suggested-- be that they would fit well within a book that explores multiple worlds. Then whole book is about expanding options, for settings and everything else. Which maybe would explain the disparate nature of the subclasses they've been exploring?
Anyway, I'm just speculating like everyone else. WotC has done a pretty good job of not giving away their plans beforehand. So I'm ready to be surprised.
You could say the same thing about the Group Patron rules in Eberron: while they are using Eberron themed organizations, the rules could fit in any setting.