I was thinking you could have an automatic harpoon, like a shark trap, however it would do less damage, deter the shark easier, use up ammunition instead of using none, the ammo would get stuck to the shark, and you could retrive it if the shark got close enough, have a clip version which would store more ammo, and another version which would have a bell buoy which would make noise, and alert you if the shark was getting close.
There should be gun parts in barrels that are very rare to find, like 1/100 barrels have 1 part, and find blueprints for making gunpowder, bullets and a crafting station, like you can do in ARK, and cannons for fighting pirates and maybe multiplayer combats. (if you do this, make PvP and EvP servers and LAN possible) Also, nig youtube channels like ZerkaaPlays and Slogoman play this game, look at their vids for feedback, and as a Promotion source.
These are great ideas. Adverse weather effects and a day/night cycle are fantastic additions. Needing to seek shade during harsh sunlight and warmth during cold nights is realistic - with rain/snow exacerbating the issue.
Scripted-events would be a great addition. As you progress, certain resources should begin to dwindle. These events would provide a way of obtaining those resources but would only appear after a set amount of play time. Here are a few examples:
1. You drift toward a derelict raft. You need to determine whether or not its worth the risk of leaving your raft to explore this new raft. Maybe there are enemies hidden on-board or sharks surrounding it. There's also the risk of your raft drifting too far away to return.
2. You land at a small island. You can explore it for resources but you risk encountering dangerous animals. The island doesn't generate important resources. It MUST be harder to survive on an island than floating on the raft.
3. You float over an undersea cavern which can be explored after dropping an anchor. There could be treasure inside but also enemies and possible drowning concerns. Maybe even traps.
4. You approach a whirlpool which will destroy your raft if you don't steer around it.
5. You're approached by a militant raft with human enemies that use fire to damage your raft.
6. You're approached by a friendly raft that has armed or unarmed traders or NPCs.
I'm not totally on-board with armor as it would likely tunnel the game progression toward reaching that safety net.
I think rain and water purification/collection system from rain would be cool, but as an adverse effect make rain put out all fire necessary items(cooking etc.) that are not under a safe platform.
I would also love to see thatched roof systems and maybe thatch pipes to carry said water from a rain catcher to a barrell?