Greyhawk is my setting of choice, and I hope we never see more than a couple of suggested rule alternatives for it in this edition. Everything anyone could ever need to run campaign after campaign in Greyhawk is already out there and easy to convert, and I really don't want to see that setting get bloated like the FR. I think the same can be said of Dragonlance, too. If you want to run a Dragonlance campaign, you almost certainly already have everything you need to implement the white/red/black circles of wizards, Knights of Solamnia, etc. All you need is a tweak to the dragonborn race and a few titles from D&D classics and you're set.
A planar book, like an updated Manual of the Planes, strongly influenced by Planescape, featuring a bunch of 5e updated planar creatures and some handy tools to flavor your planar jaunts would be great. A Dark Sun book would also be welcome, because that setting is different enough to justify it, requiring a bestiary and some class options. I think the same goes for Eberron, and both Eberron and Athas need a mystic class and psionic rules for this edition. I think an Eberron book would be a good place to detail some spellpunk/steampunk tools and systems that could be adapted to various settings and homebrews (like Zeitgeist).
As I see it, a book that detailed one or more alternative settings wouldn't suffer from the Adventurer's League problem, and wouldn't need to reprint a bunch of material from earlier products just to accommodate the asinine "PH+1" rule that AL participants somehow manage to abide. Maybe that's just wishful thinking on my part, but the last thing I want in a new D&D book is a verbatim restatement of content I've already got on my shelf.