I have a feeling Jamie's arc will carry a history-repeats-itself theme. Jamie will see Cersei become this monster - akin to the Mad King - and will kill stab her in the back. I love that Brienne prediction and a fusion of both of ours would go something like:
Brienne approaches Cersei surrounded by The Mountain and Jamie. The Mountain engages in a battle with Brienne and is badly hurt in which, Jamie has trouble seeing this event enfold while Cersei smirks snidely. Brienne reminds Jamie of all the horrible things Cersei has done and before The Mountains final swing of the sword, Jamie stabs his sister in the back, killing her. The Mountain stabs Jamie, holds him up and before dying, delivers a swift blow to the Mountain's head - with his iron hand - killing him as well. Brienne is saved and writes all the good Jamie has done in the book of knights along with help from his brother Tyrion.