Hey! love the game, it is a ton of fun and there is tons of potential!
I am super glad you don't listen to the people badgering you for multiplayer or just "make the raft bigger" in every other post. If the game as it is now, which has minimal (although fun!) content diverting your attention to multiplayer will sap your energy to deepen the content of the game.
A lot of great things have been suggested but I would like to start with adding in some realism. The raft is planks of wood yet you can build infinitely tall rafts. A lot of people enjoy the building wooden towers aspect but maybe that can be left to some kind of world builder mode where physics don't matter. I mean the game is called RAFT and it plays like "build a giant wooden island." I would suggest requiring building pontoons out of wood or floating plastic or whatever that need to be placed under foundation that is built on. Perhaps you can start on a raft, like literally a plank of wood, and you build a separate base you have to anchor to a reef with pylons or float with pontoons, that would allow people to build taller structures. Then you can build better small sailing raft like vessels. The shark would mainly harass you when you sail not when on your dock-like structure until perhaps later in the game when large sharks come. There will have to be some motivation to constantly sail. Perhaps the end goal is to find dry land? Or maybe if you keep fishing in one spot you over fish it so you have to sail to catch more fish? I like the idea of farming but not on the raft, on the base.
Finally a basic day and night cycle (suggested a million times I know), the raft moving with the ocean/drifting without an anchor, storms, swimming under water, varying depths of water, birds that steal your food and nest (eggs). From there you could add more npc sea critters varying in size and characteristics.
Thanks for reading!