Lol, whenever you referenced kingdom hearts, i remembered a failed adventure of mine, where my players found a device that transported them to another mini-plane, after they completed a specific task they would find a code to go to another one and unfold a mistery of a plane-traveler , however my players are more traditional, and wanted a whole overworld to explore. maybe you could put this in your adventure!
i allways wanted to make a sea adventure, kudos to you!