I think Ryne staying on the First would be more fitting. She's strongly tied to the place and someone needs to stay behind to help safeguard it after the Warrior of Light returns to the Source. I expect Thancred to choose to stay behind for her sake with the implication that he is living on borrowed time. Conveniently the 'time bubble' that the plot embraces means that there isn't any pressure for the writers to follow up on that until later on, if ever.