I said Planescape/Spelljammer hybrid, because I think it will come in the form of an adventure that is mostly Planescape, and then goes Spelljammer in the final chapter.
I also think there's been an interest by all the non-grognards (myself included) about seeing what Greyhawk is all about - what really is different between it and an adventure on the swords coast. I can guess at the other two, but with Greyhawk it seems like a western fantasy setting that's less developed than FR.
This is my feeling as well: a hybrid that is 90% Planescape, with Spelljammer as a variant approach to traversing the planes.
I'll probably get flack for this, but the differences aren't as huge as some make them out to be. They're both basically kitchen sink/vanilla fantasy settings, which provide the standard array of D&D options, and without a specific thematic approach (Dark Sun) or singular meta-story (Dragonlance) that defines play. I think the love of GH is mostly association and imprinting. Whether we're talking about first wave grognards (started pre-1980) or second wave (started in the 80s, like myself), Greyhawk was the first supported D&D setting. The 1983 box set was the first published setting I owned, and defined "official D&D."
Now Greyhawk does have a more sword & sorcery vibe, and of course famously (or infamously) far fewer mega-NPCs wandering around defining world events. But the difference comes down to the imaginations of Gygax vs. Greenwood, and perhaps the fact that the Realms has more of a community feel: though it still feels very Greenwoodian, it has been added and expanded by many others, while GH stayed relatively pure to Gygaxism, especially if you ignore the From the Ashes phase.
My first reaction was yawn not that dead horse again.
I'd rather get something new. I haven't had time to explore Exandria, or Theros yet but a world that based on 5e rather like Eberron was built around 3.5 would be my preference.
That being said if we have to go old give Greyhawk, a Planescape/Spelljammer hybrid, or full FR coverage.
Personally I think Netheril is probably on the slate considering the end of RotF.