Jaime will kill Cersei . . . in one form or another.
Cersei's prophecy foretells that she will die at the hands of one of her brothers, and we have our money on Jaime. He clearly recognises Cersei's cruelty and growing madness, and he leaves her for good in the season seven finale . . . but should she embrace her Mad Queen tendencies in full, he may have to come back and become a queenslayer.
Of course, if Jaime doesn't choose to kill his sister, there's also another way for the prophecy to be fulfilled: by Arya wearing Jaime's face. Read that full theory here!