Since even Blizzard now accepted that "leveling phase is annoying, let's give a free level 90 and make it even quicker" let's go one step further. Games like Everquest Next will do it, but for the expansion after WoD Blizzard could also do the same. Remove Levels. I already hear you scream: "Whaat? How can people learn to play their character?" "But I like doing quests and progressing from one area to another". Calm down - both still can be done.
If you start a new character in that new system it will be like starting a Deathknight today. You will get your skills one after another doing certain quests. So you still visit areas and travel the land.
BUT after you have all your skills all the old areas will not be abandoned and useless. Since there are no levels they can do dailys and exploring and Timeless Island like content EVERYWHERE. You love ice and snow, only play in Winterspring or Northrend. You hate snow and love the tropics? Go to Stranglethorn or Un'Goro. You prefer Pandaria, stay there...
The whole world and not just the maxium level zones will be your playground. It offers so much more for both, players and developers. You can place hidden treasures, add random events all over the place and so on. You still progress with your gear, you still get stronger with your gear but you would not craft useless gear only good for a small level frame. Raremobs will not drop loot only good for 2-3 levels.
Think about it, I think this is the way to go in the future. Sorry for my bad english (not my native language).