Elenarie: Will not happen until WoW and SC2 are older. Use logic. They will never have 2 RTSes in the market.
Breja: It was a joke. Use humor.
Also, how much older does WoW need to get? The fucking thing is almost 11 years old!
It doesn't matter. It still has 7+ million players. 6.3 is coming soon that will tie up the story of Warlods of Draenor.
Warcraft Chronicles Volume 1 was announced, coming soon, a book very similar to Book of Cain / Tyriel, that will sort of summarise everything up until the first war.
The Warcraft movie is coming that will have a more detailed look at the first war.
New expansion will be announced at BlizzCon, and they are aiming to release it close to the movie release date (10th of June) in order to cross promote.
Whispers of Oblivion is releasing soon, mini campaign for SC2.
Legacy of the Void is coming later this year, concluding the story of Starcraft 2.
The LotV multiplayer lifespan will be at least a few years after its release.
So the things mentioned above need to happen for a new Warcraft RTS to be released. Which might be never, considering that Starcraft 2 is one of the few profitable RTSes these days. The genre is completely dead, and SC2 is singlehandedly carrying it.