Yes, I know; "but WoD hasn't even come out yet and your already talking about the expansion after that one!"
Okay, now that I've gotten that one out of the way, let's talk WoW expansion #6.
Metzen said that from now on starting with Cata each expansion ties in to the next one. Therefore, we can assume that expat #6 will somehow be very connected with WoD (Just like WoD is with MoP and Mists was with Cata).
Blizzard said they want a new expansion every year (with a year and a half at the most). This means expansion set #6 will most likely be released, at the very earliest, around fall 2015. Spring 2016 is a more accurate date, though.
WoD has set a new precedent; rather than introducing a new race or new class or new profession, Warlords instead focused on one single piece of content that is WoD's selling feature (Garrison's). With this, we can now know that Blizzard no longer sticks with the formulaic new race, new class, or new profession style of content but may focus all it's efforts on a signature piece. Although , personally believe that new expansion will introduce either a new class or professsion, it may not be too far off for Blizzard to not do that and instead work on, say, total mounted combat (an example).
And finally, we know that Blizzard's seamless instancing technology (aka phasing 2.0) will be reintroduced in WoD and is significantly more powerful than before. A guess that expansion #6's raids won't have instance portals and can be accessed without a loading screen isn't too far off.