Not sure why this is an issue...? We already have temporary npcs in the forms of Lyna and the Exarch version of G'raha, so just punting an NPC out of the selection screen for specific dungeons obviously isn't an issue. I seriously doubt Ryne would just show up in Endwalker dungeons, as she has her own life to live on the First and there's no valid reason for her to just somehow traverse the Rift "just because."