I've been pondering some more, and I think it depends on what business goals they have for the new books:
A: Sell more books to existing players:
I believe that this goal means new mechanics, not new aesthetics. They'll want to give the players new tys to play with, not just new setting details. This favors the planar settings like Planescape, Spelljammer, and possibly Ghostwalk - but since what those offer can be condensed, I'm not sure any of them get their own book. A big Manual of the Planes book could encompass all three with some extra room for more Fey stuff. I can definitely see this happening, but I'm not sure if it really counts as a setting book.
The only classic setting with traction and a need for new rules is Dark Sun, since it needs psionics. If they want to sell existing players a new setting, DS is a top contender.
B: Bring in new players
I have a feeling this is a bigger moneymaker, so I'm inclined to think this is the strategy. There's two approaches I can see here:
B1: Leverage an existing IP and get those people to play DnD:
This is the point of the MtG settings, after all. I think an argument can be made for Dragonlance here - there's some number of people who read / have read DL who don't currently play DnD, and a shiny new book could get their attention. I have no clue how big a market this is, but it could be a thing. The only other IP Hasbro owns that could be leveraged this way is My Little Pony, but unless G5 is as big a hit as G4 I find this highly unlikely.
B2: Offer new kinds of fantasy, from the perspective of non-players:
Doing this involves new aesthetics more than new mechanics - they want to look for 'styles' (or looks or subgenres) of fantasy that they don't already have on their covers. The biggest gap might well be JRPG-looking fantasy (which is a ton more gonzo and exaggerated), as opposed to the more western style they've been using. However unless they overhaul Kara-Tur to unrecognizability this would involve a new setting, not a classic one. They could go for more 'gritty', but I think the ending of the GoT tv show might have tanked that for a bit. If the new Wheel of Time show revives the idea this could make a comeback. I'm not sure which setting they'd use for this. Dark Sun could be used for a post-apocalyptic and/or swords-and-sandals setting, making it the only setting to be dual-purpose - but these are not ascendant genres, so it's a risk for this goal.
So my revised list is: #1 Dark Sun, #2 some kind of combined planar book, #3 Dragonlance, massively distant #4 Kara-Tur with heavy revisions.