While we're on the subject of Roy promoting earlier, I was thinking maybe a (somewhat) lore-friendly way of making that happen would be to move the location of the Shrine of Seals from southern Bern to maybe somewhere in eastern Etruria. Then Roy and the gang could stop there on their way to either Ilia or Sacae, and Roy promotes and gets the Binding Blade. But since the Binding Blade is so powerful in the base game it wouldn't be good to give it to the player that early, so I'd say nerf it really hard, and explain it by saying the sword hasn't unlocked its true potential yet, and then they restore its full potential (to its base game stats) after 21/21x. It would be similar to how Three Houses gives you the Sword of the Creator early on but you don't get its full power until later in the game.