What if it was possible to fly from anywhere to anywhere, without having to use ships. So you wouldn't get fatigue when you fly across the sea. You could fly from Kalimdor to the Eastern Kingdoms. You could fly from Lordaeron to Silvermoon to the Sunwell and up til you're back in Stranglethorn again - like you're actually on a real globe. You could fly from Darnassus to the Exodar. You could fly from Ratchet over towards Stormwind and on your way there pass the Maelstrom and Kezan.
It would maybe take longer than the ship route, but what if they fill the sea with lots of islands, small and big, for the next expansion. Some islands would be level up zones, some would feature nice things like the Timeless Isle that promotes a more open and free gameplay, and most importantly there would be lots of hidden things in that giant space, that would motivate players to go exploring.
I'm thinking about games like the older Final Fantasy games, where you had airships and you could fly around the world and go explore. It was such a free feeling, I would love that in WoW.