5.4 is already on the PTR, and if it takes as long as 5.2 it'll be live in about 2-3 months. Just about September and almost a year to the day since the release of MoP.
Now, they finally managed to pace themselves and almost perfectly released patches in such a timely manner that it is now down to a science. Every minor patch has been released after a similar amount of days, with the larger 5.2 being released almost 1 1/2 times as long. This roughly leads to a patch every 3 months, and a major raid tier/pvp season being released every other patch.
So while the very punctual and timely patches have finally started to happen, it would be a bad time to start and a bad way to do it if they are still releasing expansions once every 2~ years.
The dead zone of time between the last patch WotLK and Cataclysm was almost exactly a year, and they heavily implied they regretted it. Patches in Wotlk were few and far between, and the Argent Tournament lasted entirely too long. The last patch of Cataclysm lasted almost as long, but since MoP released in september instead of November it was 9 months of dead air instead of 12.
Blizzard has heavily suggested switching to one expansion a year instead of two years, but I still don't believe it's going to happen.
Instead, I'd like you guys to take a look at this:
Final season of Burning Crusade: Brutal Gladiator.
Final season of Wrath of the Lich King: Wrathful Gladiator.
Final season of Cataclysm: Cataclysmic Gladiator
Final season of Mists of Pandaria: Grievous Gladiator?
Disregarding the obvious fact that Burning Crusade was the first expansion to feature Arenas and they missed the "Burning Gladiator" oppurtonity, Grievous Gladiator being the last season of MoP doesn't sit right with me when the theme of this expansion (and the name) lead to the obvious "Mystical Gladiator" title that many people assumed would be the final season of MoP.
tldr: Garrosh escapes in 5.4 and leads a Sha empowered army or another threat emerges from him dying, MoP goes on until 5.8 next year when he is truly defeated, and the final PVP season "Mystical".
Either Blizzard does that, or we have another long year of no new content. You decide what is more likely.