My crazy theory is that she will be both a Jedi and a Sith Lord. I think it's inevitable that she'll at least get tempted by the dark side, otherwise there don't seem to be any obstacles for her as a character. Perhaps she will truly "bring balance to the force", by ending the animosity between the Jedi and the Sith. Destroying one side of the force isn't exactly what I would call "balance."
Rey shouldn't bring balance to the force. They need to stop retreading on previous themes. Anakin Skywalker brought balance to the force. He was the Chosen One. Let Rey have a different destiny.