I really hope that it is an Akaviri invasion. But the teaser clearly said they were already here, as in, not a return but an awakening. I am inclined to believe it is the Aedra attempting to escape the mortal plane that they were trapped in by the Lorkhan when he created Nirn.
From the PGE:
"The high crafts of daikatanas and dragonscale armor came from Akavir, as did the banners and military dress of Septim's shock troops, the Blades. The Red Dragons that have come to represent the Empire and the Imperial City were originally Akaviri war mounts. Akaviri surnames are rare and prized possessions among the Cyrodilic citizenry of today, and there are trace facial features of the Akaviri in many distinguished Cyrodilic families. Some colonies of "true Akaviri" still exist in both the Empire and its border regions, but they are named so only for their practices and customs than for the purity of their blood."also
"The Second Empire is divided into two stages: the Reman Dynasty and the Akaviri Potentate. As mentioned in the text, after the Akaviri raiders had been defeated, Reman recruited many of them into his service. Later Cyrodiils traditionally kept a House Guard of Akaviri, and the Emperor's chief advisor, the Potentate, was usually of Akaviri descent"That explains how dragons and Akavir are related (let's not forget that the original invasion came through Skyrim, and happened to lead to the rise of Reman's Second Empire, which the Tsaesci were able to eventually take control of to rule Tamriel -- coincidence, I'm sure.). So "their defeat was merely delay" is clear enough, if we're talking about Tsaesci.
But what about "they're already here"? Well, the obvious answer is that there is an invasion that the speaker's audience is just starting to prepare for, unaware that it is already well underway. But let's say it does mean that the threat was dormant in Tamriel the entire time, either waiting to be awakened... or waiting for the right time to strike. What about this:
"And in the days of interregnum, the Chim-el Adabal was lost again amid the petty wars of gone-heathen kings. West and east knew no union then and all the lands outside of them saw Cyrodiil as a nest of snakemen and snakes. And for four more hundreds of years did the seat of Reman stay sundered, with only the machinations of a group of loyal knights keeping all its borders from throwing wide.
These loyal knights did go by no name then, but were known by their eastern swords and painted eyes, and it was whispered that they were descended from the bodyguard of old Reman [note: the passage from the PGE above suggests the Reman dynasty used Tsaesci bodyguards]. One of their number, called the Chevalier Renald, discovered the prowess of Cuhlecain and then supported him towards the throne. Only later would it be revealed that Renald did this thing to come closer to Talos, anon Stormcrown, the glorious yet-emperor Tiber Septim; only later still, that he was under instruction by a pig.
Long glory was wife to the all the knights of the dragon-banner, who knew no other and were brothers before beyond many seas and now were brothers under the law named the blade-surrender of Pale Pass. And having vampire blood these brother-knights lived for ages through and past Reman and then kept guard over his ward, the coiled king, Versidue-Shaie. The snake-captain Vershu became Renald became the protector of the northern west when the black dart was hooked into Savirien-Chorak."It could be misleading conspiracy theory nonsense (TES doesn't like to give objective lore, which I love), but the implication is that the leadership of what would eventually become the Blades are in fact Tsaesci who live still from the time of the first invasion -- ageless but with the appearance of humans. And the reference to the time of interregnum and failed empires... sounds an awful lot like Tamriel after Oblivion, and especially after the events of The Infernal City, yes?
Apropos of nothing, one of Daggerfall's books tells the story (probably fictional, even within the gameworld), of a hero named Lysirius (somehow associated with the Daedric Prince Boethiah) who manipulated a powerful dragon into destroying an army of Akaviri.
Akavir is four countries, who would never band together for anything. Saying Akavir would invade somewhere is against the established lore.
Unless one of the countries prevailed and subjugated the others.
EDIT: My personal conspiracy theory: considering how handily the Akaviri whooped Uriel V, the Skyrim invasion was never intended to serve as a true conquest -- it was intended to terrify Tamriel into uniting under a single Empire, which the Tsaesci would then be able to control from behind the scenes. Much easier to subjugate people when they don't realize it happened. The actions of the Blades can be seen, in this light, as a means of protecting the Empire itself, as an implement of Tsaesci rule re-established under Talos.
In fact, Talos himself may have been a Tsaesci imposter, or at least may have had one in the Underking. Consider "and there are trace facial features of the Akaviri in many distinguished Cyrodilic families" and "Later, many Nords could not look on him without seeing a dragon.". Snake, dragon, a reptile is a reptile.