Bran just realized he has this "Warg" ability, which he can use to control the minds of animals and humans too to some extent. I havent read the books so its not a spoiler, I believe Arya will join/trained by the faceless assassins, brotherhood without banners and maybe The Hound to become a perfect assassin. Those two with Jon Snow and Sansa as an insider can get the job done but judging from how the shows have been, I wouldnt be surprised if they kill rest of the Starks and Snow next week and finish them off altogether.
Thats not as big as dragons or witches or black magic but now that the Starks are not seen as a threat anymore, people(Lannisters, Greyjoys & Freys) will definitely underestimate and even forget about them, now they can attack from the shadows and extract vengeance as it is easier when they are not on the enemy's radar.