Bookmarked for pure zomg.This is going to be a lengthy post with some references to tin foil hat articles on wow insider and such but bear with me. I may go back and edit the structure of this post as well to make this easier on the eyes. Also feel free to correct any mistakes in my recollection of the lore, may be wrong with a detail or two but I'll give it my best.
So in the 5.3 ending quest for the legendary, Wrathion consumes the heart of Lei Shen that has a massive concentration of Titan power, stolen from Ra-den (The titan watcher who was charged with protecting the Engine of Nalak'sha.) During his titan power induced vision, he sees thousands of stars out in the cosmos and mentions that Azeroth is unique. As the OP mentioned, Wrathion states "WE HAVE FALLEN. WE MUST REBUILD THE FINAL TITAN. DO NOT FORGET." This is likely a message intended for all Titan constructs, and it's interesting to say the least, with the mention of having fallen (at this point nearly every titan construct we've run into has succumbed/failed their job) I want to focus on the "rebuilding" of this final Titan.
Creation of Azeroth:
Azeroth was originally ruled by the old gods and the elementals, but at some point Azeroth attracted the attention of the Titans, a race of all powerful beings that wielding the powers of creation and order. Why? Why would Azeroth attract the attention of the Titans? Although they are all powerful, why would the Titans risk war with the Old Gods/Elementals over a single planet? I believe it's because Azeroth meets the rare requirements for the creation of a Titan. The Old Gods were defeated and imprisoned beneath the ground and the Titans set about creating life on the planet, creating watchers/titan cities/aspects and bestowing the seed of creation on the planet to create the new Titan deep within the planet. Why would the Titans imprison the Old Gods instead of killing them? It always falls back to the corruption being so deep that Azeroth would be destroyed along with them, but why would you be concerned about a random planet (The Titans create life on thousands of planets mind you) when you had the chance of destroying pure evil/chaos? Because again, the planet met the rare requirements to be suitable to create a Titan.
In Wrathions statement as well it mentions rebuilding this Titan, but why would we have to rebuild this Titan? Wouldn't the Titans set everything up in order for this Titan baby to naturally be created/grow and eventually depart the planet to join its brethren? Perhaps it was, until the misuse of the Well of Eternity's powers by the Night Elves which attracted the Burning Legion. Now the reckless misuse of the Wells power apparently sent out ripples into the Cosmos, eventually garnering the attention of Sargeras himself but why? Why would Sargeras care about this planets power use in particular? The Burning Legion is pure destruction, a wave that consumes all planets in its path so why would he suddenly move towards Azeroth? The Well of Eternity was a massive body of mysterious liquid which allowed the night elves to use magic. It's even said that the Night Elves evolved from Trolls who had resided on the shores of the well and mutated over time. But what if the Well was meant as a power source for something else? What If the well was the power source for the creation of this new Titan? With the Wells Destruction, the leaking of the Old Gods to the surface and the Cataclysm, it's no wonder this Titan has to be rebuilt, he's severely damaged. Now this job would fall upon the Titanic watchers/constructs, but they've succumbed to the curse of flesh and/or forgotten their initial assignments. So what do we have? We have a damaged, possibly corrupted inactive Titan inside Azeroth, waiting to be completed.
Now Sargeras has been interested in Azeroth from the moment he sensed the Wells ripples in the cosmos, even after being defeated multiple times and at this point having no physical body he continues to push onward, rumored to be returning with the Legion in the next expansion, why? Because if Sargeras is able to get his hands on Azeroth, this Titan Engine with its Titan Infant could be influenced/corrupted by Sargeras himself, or even worse Sargeras could once again have a body, destroying what soul this Titan could have possibly had and replacing it with his own.
This is a rough theory, and it could use some polishing but this was the best I could do on short notice.