I have a pretty awesome/horrible idea: on her trek to The Wall, Brienne comes across Arya and Hound, and she fights Hound to the death. Hound kills her and takes her sword. This death could serve the deliciously terrible purpose of poo-pooing all over the so-far-ultimately-unnecessary oathkeeper storyline. It's one of the few ways the writers could use a surprise death without screwing things up.

I don't think that will happen though. Most likely Brienne ends up killing a bunch of wildlings in the soon to come epic struggle on The Wall