Two things are both (potentially) true at once:
One, it is far more likely that the "revisit" is FR over GH, for reasons others have stated - but also the simple fact that the majority of the current player base knows the Realms, and only GH through Saltmarsh. If you're looking at likely buyers and the product line as a whole, FR makes a lot more sense.
Two, WotC is (sometimes) full of surprises. Who predicted Ravnica would be the first Magic setting? A few years ago, who would have thought Spelljammer would be revisited? Etc. It is also theoretically possible that they'd publish GH as a passion project.
I also think Planescape is more likely than Dark Sun, with everything else far in the distance. That said, if the battle games do well, I could see them resurrecting Birthright, but probably not until after the 50th anniversary. Meaning, there are limited slots in 2023-24, and there's almost zero chance that Mystara or Birthright or anything else would come before PS or DS.
So my guess for settings:
2023: Planescape, New Setting, Magic Setting
2024: Forgotten Realms, New Setting