With WoW's 8th birthday around the corner, how long until a sequel?

Today I decided to give finally up on Diablo 3 for a myriad of problems I have with the game; I've been a huge fan of the franchise since its earliest days, but the current game doesn't even touch Diablo 2 for quality or gameplay and I'm not the only one who agrees -- with a massive drop off in player numbers affecting gameplay further. So I'm looking to an alternative to get my RPG fix on my Mac rather than console. I've tried MMOs before and they've never really appealed to me as they feel too open without direction in regards to story, so Diablo 2 for me was a good alternative with eight-player games. However, I have tried WoW before on a trial about five years ago and I have to admit, I liked it -- in fact, a little too much -- where you log on and the next thing you know, hours have passed so quickly, which is why I didn't continue with it! I didn't want it taking over my already busy life; things have changed in that respect and I have more time for gaming, so would consider trying it again, but I think it's somewhat dated.

Its eighth birthday is just around the corner with a new expansion having recently been released, but shouldn't Blizzard be looking to call time on it and develop a new up-to-date, and even bigger and better sequel rather than releasing more expansions just to boost subscriptions that will inevitably begin to drop off again? Any word if Blizzard's even got this on the drawing board yet because, of course, it takes years to develop a game. It took about five years or so to develop the mess that is Diablo 3; anyone think WoW will still be the cash cow that Blizzard relies on in five to six years' time without a sequel?

For now I'll try Torchlight 2, perhaps League of Legends, or maybe Guild Wars 2. But if a new WoW were in development, it would have me very interested! :)