Wow, that's tough. I have a massive love for Planescape, so it would be great to see that back, as well as Eberron. Rounding out my top 3 would be an entirely new setting. But like others, pretty much all the old settings would be great. In particular, an updated Spelljammer could really stand out at this time.
However, what I really, really want - is 3rd party support for campaign settings. License them out so that we can get full product lines rather than just a player's handbook and DM's book/adventure. The best aspects of the earlier settings are their overwhelming richness. Just a couple books doesn't nearly do justice in conveying a rich setting, especially if they create a new 5e setting.
Imagine if Planescape was only the Planewalker's Handbook and Dead Gods. Both are awesome books, but nowhere near enough to capture the setting as much as it deserves.
Unfortunately, I don't see that being supportable enough for WotC, yet would be awesome product lines for the right 3rd party publishers. Fans get the settings they love. WotC gets profit from a licensee without the effort or risk of publishing it themselves. 3PP gets access to valuable IP with built in audience. Hasbro sees D&D IP being spread and tested among the fans to see what catches and warrants possibly pushing into other markets. It's a win-win-win-win!