Also an idea for you if you can make it will be an immense work for campaign;
Make an island that have a castle which can be accesible via boat or a ship. Like Baldur's Gate or Pillars of eternity or other this kind of game types in rpg.
You can save the castle and own it. (Maybe a story based saving ? )
You can upgrade your castle building, hire mercenaries to defend its grounds, Can collect taxes, and while you are away there can be a script counter for attack on your castle can be invoked while you can defend it by your hireable mercs & more stuff like that ?... Can be fantastic