Here's my wall of text list of suggestions:
- Shark Revamp; sharks should get more aggressive and more destructive as they lose more health. Also, sharks' spawn rate should increase as the game progresses (it would hit a cap eventually). Sharks should also make more noise.
- Floating Items I; the number of items that float around the raft should gradually decrease over time. It should never hit zero, but shouldn't remain at a static high either.
- Floating Items II; items that float under the raft should sink. This not only creates a greater sense of urgency in gather items but would prevent people from using planks to create a barrier around nets and thus make nets more valuable.
- Net Capacity; on the topic of nets, they should have a limit to the number of items they can hold. The player would be able to hear sounds indicating that the net is being strained, and if they choose to ignore it, the net would break and all items will sink.
- Water Hazard; placing too many items on a raft with too little base surface area should cause the raft to sink (not necessarily tip over, but slowly dip into the water). Too much sinking would cause the bottom floor to collapse and all items on it to be lost.
- Fire Hazard; since it is a wood raft, inventions with fire should pose a chance of creating a raft fire. Additionally, standing near the said inventions should carry a risk of accidental burns.
- Durability; all items (including the raft itself) should have a durability. This makes sense since the environment is constantly damp and salty. This would also make collecting resources much more important.
- Food System Revamp; make it so that both plants and fish must be eaten in order to stay alive. This could be done by having two food bars instead of one. This would also give players an incentive to farm.
- More Plants and Fish; create different plants which could be grown and different fish which could be caught. Certain types of either/both would be poisonous (see below) and it would be up to the player to learn which ones are.
- Special Food Items; barrels should carry special foodstuffs like chocolate or cola. These foods would give some sort of speed/strength buff, or perhaps just serve as a rare little treat for the player to find.
- Poisoning; food items should carry a risk of poisoning (which inflicts DoT and perhaps messes with the player's FOV and movement). The risk should be much greater for raw food items.
- Hypothermia; nighttime should carry the risk of hypothermia. This risk would be reduced by standing close to fire and increased by going into the water at night. Fatigue should also set in much faster at night.
- Hyperthermia; as the game goes on, standing in the sun for increased periods of time would cause the hydration bar to deplete faster. This would give players incentive to build a second level (or at least plant a tree) for shade.
- Heavy Injury; certain acts (spearing a shark, for example) should carry the risk of sustaining a heavy injury (which inflicts a large DoT). The injury would take long to heal unless the player has a...
- Medical Kit; craftable item (also obtainable in barrels) which counteracts the effects of the poisoning, hypothermia, hyperthermia, and heavy injury. It would also replenish a small amount of health.
- Diving; going underwater should be possible, and retrieving sinking items while diving would also be possible. This would have the fatigue bar see more relevance. Fatigue should set in faster while under water.
- Sleep System; since there's now a day-and-night cycle, perhaps make it so that some form of sleep is required such that lack thereof would cause fatigue. Sleep should not be possible unless the player has a roof over their head or is in a...
- Bed; allows players to sleep, but also lets the player recover from poisoning, hypothermia, hyperthermia, and heavy injury at a faster rate. I say 'bed', but it really should be more of a sleeping bag.
- Auto-Craft Machines; craftable machines should be implemented which automatically craft simple items (ropes, nails, etc.) over a short period of time. The machine would take raw materials out of your INV and give you the finished products with "E".
- Hand-Held Torch; a tool which carries minimal fire hazard and lights up surroundings when held in the hand. The torch would be lit by holding it close to a fire and would extinguish if the player goes in the water with it.
- More Hostile Entities; be it mobs, storms, tsunamis, strong winds, or et cetera, there should be more sources of danger to keep the player on-guard and diligent.
- Rifle; a primitive rifle should be craftable so that sharks and other hostile mobs (see above) could be killed faster and from farther. It would require depletable but craftable bullets to shoot (obviously).