Azuma is currently lacking it's lightning deity due to Fubuki's brother being a moron, so there technically is a vacant position that Sovereign Kaguya/Subaru could have their mortal love interest fill by deifying them. I'm not sure if they can just create new positions on the spot though, since we know gods are sustained by having a populace to believe in them and domains to preside over in Azuma's territory. This is repeatedly suggested to be essential (minus for Hound I guess, somehow he's able to stay super strong with no village to love him. Maybe being feared works for it? Idk.). It'd be nice if they went into what being Sovereign entails more rather than just having it be a last second add-on at the end of the post-game dungeon.