This is not the shadow of Drogon or Rhaegal arriving at King's Landing that Bran saw, but the Night King riding Viserion whilst his army lays siege to Winterfell.
There is no Valyrian steel or dragon glass at King's Landing. They are fucked.
The Night King will obliterate King's Landing with dragon fire and will raise the 500,000 strong population to fight in his army.
He will then march his army north.
Winterfell will be succesfully defended in Episode 3.
The final battle between the living and the dead will take place at the Trident;
In the books, it is heavily foreshadowed that Jon is the legendary hero, Azor Ahai, born again;
Jon was born under the bloody and legendary sword of Ser Arthur Dayne, Dawn, which has a red star on the pommel;
In ASOIAF legend;
Now that Dany and Jon are in love and Dany is clearly a tyrant, I think it is sadly obvious that Jon is going to stab her through the heart with Longclaw, which will become the mythical sword Lightbringer. Jon will defeat the Night King and the army of the dead. But not without cost.
The dead shall be reborn. I believe Jon will end up as a Night King. I don't know how, or why. The Starks have the blood of the first men, and shared land with the Children of the Forest. Jon is half Stark. He's going to make the ultimate sacrifice for the realm as he has sacrificed himself throughout the entire series.
The throne I think may go to a legitimised Gendry Baratheon and his eventual queen, Arya Stark.
I also believe Cersei will escape the burning of King's Landing and will flee but before this season is over, Jamie will strangle her to death.