Reddit is on top of this, obviously. User NegativeKarmaSniifer speculates that Bran will whisper into the ear of the King Aerys II Targaryen and transform him into the Mad King -- a fuck-up worthy of George R.R. Martin's cynicism. I'd like to propose a happier twist, one that revives our bracket favorite and speaks to something grand at the same time. Imagine Bran willing himself back to the Tower of Joy, or even the Stark family's early days at Winterfell, to deliver Ned a Back to the Future-like warning: "Winter is coming." If Ned avoids King's Landing, he can reveal Jon Snow's true identity, unite him with Daenerys Targaryen, defeat the incoming White Walkers before they have the upper hand, and restore Westeros to its former glory. It undoes the entire show, but hey, that's the cost of time traveling. Never stood in Lost's way.
Or maybe the Three-Eyed Raven is right. If Game of Thrones spins a political message, the time travel element teaches an essential lesson: learn from the past, don't try to undo it. Tragedy entombs Bran's greensight ability. Any attempt he makes to correct history's dry ink could butterfly-effect his own existence. Or worse, it could keep Westeros in an infinite loop, stuck in time, forever at war (see Stephen King's The Dark Tower for reasons why this isn't that insane). Part of me wonders if Bran is destined to run parallel to the rest of the show, fuddled by his save-the-world-quick scheme and destined for the Three-Eyed Raven's fate. The exact fate, even -- there's not enough evidence in the show or book texts to suggest Bran and his mentor aren't one and the same. All too weird? Never forget that Martin writes just as much sci-fi as fantasy.