If they focus on settings, I think it is indeed possible they will increase the number of books per year. That's because generic books like Xanathar are more easily seen as potentially useful by all gaming groups, while setting-specific books have a smaller target.
Then, all 5e supplements so far featured a blend of fluff and crunch. What I wonder, is what kind of crunch they can fit into setting books, that can really shine as relevant and iconic to such settings. For this reason, I think Dark Sun is the most likely fantasy setting to see a whole book about, at least because psionics is such an integral part of it, and will make such book really "heavy" and important.
Birthright could contain a few very interesting rules modules, but the setting itself has a bit fallen into obscurity through the years. I don't really know, but I think it never had the same popularity as other settings.
Rokugan requires completely different races and classes, and can benefit from a few small rules modules too, but is a no-go because WotC doesn't own the IPR anymore.
But Mystara and Greyhawk? I am not really familiar with them, but at a glance they are such "normal" fantasy settings that I can't imagine how their settings books can have the equivalent weight of Dark Sun. If they do a Mystara or Greyhawk supplement, it'd probably be similar to SCAG, which featured only a small amount of fairly generic crunch (part of which is already reprinted now). I don't see much compelling reason for buying those books for the fluff, when you can buy their original fantasy setting books which are a lot more valuable in terms of fluff (just compare the 5e SCAG with any older edition's FRCS book).