I think it was intended to be the end game, but virtually every game of thrones fan can see it coming...so wonder if that will change things.
In some ways, I wish they'd waited till the books were finished before making it (well, no I don't because I think they've done a stand up job on it so far despite the deviations)
I think now endgame will be the destruction of westeros; the white walkers and their undead army will invade, Dany will ride in with the dragons (and possibly team up with JS and a third character) and end up having to destroy westeros to save it and end up 'queen of the ashes' so to speak. I do think bran will have a big part to play.
Jon will die properly, I wouldn't be surprised if he dies this series in a 'hah! You all thought he was back for good, now look!' twist.
I think Young Griff (or a character based on him) will appear this series; I feel they are trying to avoid moving the story on too much ahead of the books so I think they will 'go back' so to speak and incorporate him in some way - maybe rescuing Dany form Vaes Dothrak?
I think this series will focus on Ramsay and Jon Snow in an interesting but ultimately pointless storyline as well as the flashbacks to the Starks...all interesting and new to us, but I don't think things will move as dramatically as they have and the events in this series will have little bearing on the endgame.
Which seems odd as the seventh book won't be released until the 2020s most likely so it makes me wonder if GRRM has a real shocker planned for the end of book six that they want to be in the show but don't want to show it ahead of the book.