Note: This comment contains spoilers for the ending. If you have not played the game yet, binge it in one go and come back.
OK. I just came back to this game after 9 months, and it is amazing as ever. It is really only missing two things:
Ability to make choices that progress the story.
A satisfying ending.
Obviously, the former would probably detract from the game's experience (along with making it massively harder to code), and the latter is subjective, but I have a suggestion that I'm angry I didn't think of sooner: Settling Down.
Basically, the idea would be that on every island, after completing the main quest, you would have the option to settle on that island, and end the game with an epilogue cutscene of Curly living there.
"But wait!", I hear you cry, "what about the letter!?"
After the cutscene ends, the camera pans over the letter. It reads,
I hope you have had a long life full of adventures while looking for this letter's owner, but that was not what you truly needed. You were bored on that island of yours, but now you got to live life to the fullest. You had lots of adventures, explored lots of places, and I couldn't be happier for you. Wherever you are, once you opened this letter, I hope you are as content with my life as I was.
Roll credits, player cries.