Oki here is the wacky theory!
As pr. Pattern from Blizzard I find it very likely for them to continue the same pattern they have hold so far.
What pattern you ask? Allow me to demonstrate my dear comprade(s)!
Classic wow: The whole arc of stories got tied up nicely, but then Naxx came! It was kinda out of place, but non the less enjoyable.
BC: We finished the whole story of BC with the Kill of Illidan, yet SWP came (I might add the plot twist to make SWP fit in, was indeed well written and felt very natural)
But non the less I am sure it was not intended to be included from the get go, the simply needit another raid for the expansion to last longer.
Now here we are. Wrath of the lich king, the final showdown with Arthas, the final boss in this expansion. Or is it? If blizzard follow the same model we should see another raid instance release.
That's by 2 cents on the matter cheerio and have a good day!