I know that Blizzard don't want to create new capitals, cause they don't want to scatter players between different capitals, cause they also don't want to have another Silvermoon, or another Exodar. But it would be cool to change a little bit.
Maybe they could destroy old capitals for X reason and make new capitals, in order to renew the game, also there is some other possibilities : what if Exodar was repeared ? We could use it as a flying capital, on a future expansion concerning the burning legion, implying to travel in the Twisting Nether.
Destroying capitals wouldn't solve the problem. The main reason the other capitals are unused is because Blizzard put everything in Stormwind and Orgrimmar. They could make some amazingly beautiful new cities, and they'd still be mostly empty if they didn't have everything Stormwind and Orgrimmar do.
Having the Exodar as a new capital flying through the Twisting Nether is an interesting idea, but I think that if they were going to use an existing ship for that then we'd be more likely to see Tempest Keep be used than the Exodar. Tempest Keep still works, and it's probably empty now.
I'd love to see new capitals even for Goblin and Worgen, but they do need to bring every capital up to Org/SW standards, including Shat, Dalaran and Faction Shrine in Pandaria, that way people can just go where they want. As well as bringing Blood Elf and Draenei Areas out of their own zones and attach them to the main continents. Because I would probably stay in the Goblin Capital or Silvermoon if they were.
Yeah, the Exodar is up and running again according to Velen's short story.
"Reminds me of a dog. You stare at it while it's playing outside, running around in a circle, then stopping up, wagging with a wide "grin", jumping around until you call for the dog. Then it lowers its tail because it think it's done something bad and then you throw the ball, seeing the tail go straight into the air in bliss! ^^" - Gehco