The Arya Stark interpretation of the valonqar theory has existed since the young wolf earned her stripes as a faceless man. Basically, if Jaime dies in the Great War, the event would allow Arya to steal his face, travel south to King's Landing, and enter the royal chambers without raising suspicion. To Cersei, it would appear that her younger twin brother betrayed her just as the prophecy foretold, but the truth would be that Arya made good on her promise of retribution. The added bonus is that the Queen's top guard, the Mountain, also sits on Arya's list, so wearing Jaime's face into the Red Keep could result in double victory for the young Stark.