Game of Thrones will have a season 8, confirmed by HBO programming President Michael Lombardo during a Television Critics Association’s press tour. When the reporter asked if season 7 is going to be the last as show-runners have repeatedly declared in the past to hit the ‘70 hour’ mark, he said “Seven-seasons-and-out has never been the [internal] conversation. The question is: How much beyond seven are we going to do? Obviously we’re shooting six now, hopefully discussing seven. David Benioff and Dan Weiss (D&D) feel like there’s two more years after six. I would always love for them to change their minds, but that’s what we’re looking at right now.” The major actors of the show have been signed only till season 7, but the show-runners are committed to sign them again for possible season 8.
So, what will be its impact on the story? One line answer would be – they can spend the big portion of 8th season on the “War for the Dawn”. The good news for D&D is that they would have more time to take the important storylines and plots to their logical conclusions. For instance, Dany is still stuck in the East, she has’nt even captured the entire Slaver’s Bay (an area on the Southern coast of Essos) and now she is surrounded by a horde of Dothraki warriors. She is nowhere close to invading Westeros and sit on the Iron Throne. Another example would be the amount of time required to set up Arya Stark to kill host of people to avenge her past. Till season 5, she has managed to kill only one person off her list “Meryn Trant”.
There are also possibilities of a GOT prequel after the original series is over, but there is no official communication yet in this regard. It is just speculation and hearsay at this point.