So, the Akaviri invade Tamriel because they knew what the Nord heroes did. Because they knew that only a Dragonborn could stop the Worm A.k.a Alduin The World Eater. They wasn't looking for Reman, or a Septim. This Dragonborn emperors never knew what power was in their blood.
But the Akaviri knew, knew everything from the begining so they created the Dragonguard, that was allowed to protect the emperor and his line of succesion so the day when the World Eater returns this Dragonborn emperor could save the world. But they failed.
And the gods, Akatosh, gave Mortals another chance. And a new Dragonborn was born.

So,

TES III, TES IV, it's just about the prophecy of the Dragonborn of TES V. The brass tower, the dragonborn emperor loses his throne. Everything so we can watch the final Dragonborn complete his destiny as savior of the world.
So, why make another TES, sure they can start where the story of the Dragonborn ended. But a dragonborn is no longer needed. I hardly believe that the Akaviri invade again to protect him in case Alduin returns again at the end of times. This Dragonborn is the end of all the prophecies together, is the end of all the story of Morrowind and Oblivion.
In fact, in Alduin's wall we can see "The brass tower" and an Oblivion gate. I believe that Bethesda didn't have this script since Morrowind or Oblivion, it is obvious that they reshaped History so it can look that the last Dragonborn is the end of everything.

And i'm sure, that the hero of TES VI will reshape History again and it will have a prophecy (that no one thought to look for before) that will reshape everything.