Tho to be perfectly honest. I would love a flying islands themed expansion (main island being on the back of a giant dragon? = dragon isles). Flying unlock-able in the first patch (x.0), grind equal to Pathfinder part 1 (complete launch content once to unlock flying).
Floating islands would look much more island like than Outland. And with the new skybox tech they could make it seem like we're flying over different parts of Azeroth.
Change flight to be based on weather (a bit slower in thunders, normal in rain and cloudy, faster in clear skies). Not by much, 10% max just for flavor.
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As much as I would like it and not mind it, I already see forums crying about that like they do for just about everything.