For me personally, I would still like to see Dragonlance and Greyhawk come back. Yes, I know they seem to some to have an overlap of style with FR, but the story was the thing for me on these. It would also be cool to have a compendium-style book of some of the basics for each of the settings, and how to run them using the older product (which will be available in e-format at that point). Sales of the online product and surveys could tell WotC which settings we are using, which would let them gauge which ones to give their own setting books for 5e.
As a reminder, here are some of the settings that were official releases in the past & Present:
Greyhawk, Blackmoor, Known World/Mystara/Hollow World, Forgotten Realms/Kara-Tur/Maztica/Al Qadim, Dragonlance, Lankhmar, Ravenloft, Spelljammer, Dark Sun, Planescape, Red Steel, Birthright, Jakandor, Council of Wyrms, Eberron, Nentir Vale
Just roll some dice.
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