Since Bran has access to weirwood.net, what's to say he can't just tell all the "good" principal players what to do, when to do it, and how to do it? Can't he just make sure that Cersei falls, the Others are defeated and the North survives? We know he has very limited ability to influence past events but what is his presence in the "now"? Where do his motivations lie? Does his power to see the future enable him to "change" that future? How many permutations of the past/present/future can he see?

Storylines always get tricky when dealing with time travel since there's always some sort of silly way to wriggle out of dire situations. I have no idea what Bran's endgame will be in Martin's book but I really hope they handle it correctly in the show.