Build outside farm? More main story content, even if only miniscule and for immersion purposes? Some new dungeon? More romance options? Crossover with Terraria? More uses for Lewis' pants? Making NPCs divorce, so you can marry their spouse (something like a reverse friendship system with negative points, for the ones affected)? Sell your empire to buy a luxurious mansion in Zuzu City option? Become a Joja shareholder and receive income without breaking a sweat? Become the new president in Lewis' position and micromanage the entire community, or even the Governor of the region himself? Play in a movie that the movie theater is going to feature (like Shane's commercial on Joja cola with Clint, Emily and us)? C.A. appear in game briefly and have some screen time with the player?
You should be able to build on Ginger Island (with higher costs).
You should also be able to build a 2nd greenhouse / upgrade greenhouse (lvl 0 - 12x10, lvl 1 - 20x16, lvl 2 - double decked - lower deck like lvl 2, upper deck with 12x10)
Also you should add a bounty system similar to that in GTA, which increases (1 star = 100 bounty level) if:
Your or your animal's name is set to contain item codes (permanent 0.5 star)
You throw rocks from slingshot at others (+5 bounty)
You try to chop down trees in town (+25 bounty)
You hit the door tiles of closed shops with tools (+0.5 stars)
The more bounty, the slower the increase of love of gifting, less luck, more negative comments, and later there should be a police by Mr. Qi that hunts you and if you get caught, you pass out like if you glitch yourself to the summit despite unfinished perfection tracker.