I'm surprised so many Greyhawk votes.
Dark Sun is far and away the most unique and interesting setting still needing updates for D&D 5e after we're done with Planescape.
Though I wouldn't be opposed to Birthright, and Council of Wyrms re-set within Io'mandra would just be AWESOME.
And Nerath / PoLand / Nentir Vale / Bael Turath / Arkhosia / Karkhoth / Iron Circle / Vailen would be fun to revisit, though its inclusion of classic Greyhawk locations and adventures and alternate cosmology / role for the Raven Queen makes its compatibility with both Planescape and Spelljammer very awkward…