Without the power of the Amulet of kings Attrebus Medes does not hold the power to banish the dragons from Tamriel.
The player is sent to Skyrim to help the shattered Empire regain control of Skyrim after it collapsed. Somewhere along the way dragions somehow are introduced into the game. The player discovers the history of the Empire through artifacts of Alessias etc. until he travels to the Aedric plane to reforge the Amulet so that Attrebus can defeat the dragons.

In the background of the trailer you can clearly hear the Nords chanting
FOR THE LAND FOR THE WULF FOR THE GODLESS ONE.
Wulf was the reincarnation of Tiber Septim at Ghostgate in Morrowind so maybe he will guide the player through the game as Azura did in Morrowind or Ria Silmane in Arena.

As for the Dragons return to Tamriel, maybe a Ka'Po-Tun invasion. I believe their ruler can shapeshift into a dragon.
In reality the Dragons were allied to the Aedra so it wouldn't make sense them invading Tamriel but who knows.