I'm definitely enjoying this so far and the mechanics are pretty solid as a base.
Things I think would be good:
Day/night cycle and character endurance, requiring sleep to replenish.
Along with that, naturally, crafting of a bedroll and with crafting advancement, a bed.
Night should be pretty dark, creating early game disadvantages, so you'd want eventually want some lighting via torches. To craft torches, you need the ability to get oil from fish and palm kernels. That would require crafting a more advanced grill to collect the fish oil and some sort of press for the palm oil. Then planks, thatch, and oil to craft a torch. In early game, purifiers and grills (when operating) would provide a small circle of lighting.
Weather, in particular rain and wind, varying from gentle to a decent storm. Enough of a storm would drive the shark underwater (away) during it, but counter that by creating the potential for storm damage to the raft. Everything would need to be repairable and destructible with the axe, allowing for the decision to repair or replace.
Roof/canopy sections for protection from rain. Rain would prevent purifiers, grills, torches, and any other fire-using structure from functioning, unless protected by a roof/canopy. Similarly, enough wind would do this as well, unless shielded by a wall.
Get a potato plot or two up and running, you can put your fishing pole away and forget about it; especially with the occasional addition of shark meat. So...
Composting. Crop plots and tree plots wouldn't just come with free dirt. You craft an empty wood plot and then have to fill it with a required amount/mix of organic material (potatoes, thatch, fish). This decays over time and, when ready, then you can plant. Alternatively, crafting a compost bin and requiring compost as part of the crafting of the plot.
Crops require watering. Similar to the player having a water level, so would crop plots; if it runs empty, the crop/tree dies. Water would be added either through manual addition of water via cans of fresh water or when exposed to rain.
Resources somewhat less plentiful or a lot more uses for them. - Once you get 8+ nets set up, you can just put your hook away and forget about it.
Durability for nets, along with the already present ability for a shark to do damage, catching materials does a small amount of damage for each item snagged, requiring them to be repaired/replaced.
More uses for scrap metal. - It's really in excess, I throw so much of it over the side.
Scrap uses - Tool repair/durability. They require repair or replacement because they wear out.
Reinforced foundations and other structures. Not metal versions, but tougher versions that utilize scrap in construction and repair, along with the base materials.
Water collection bucket/trough. This would be for collecting rain water. This remains balanced by the frequency of storms, maintenance/repair, and requiring crops to use water.
Crafting of more advanced items, (like an oil collecting grill, an oil press, a bigger water purifier, etc) would require some sort of additional tools or crafting bench.
Fire-using structures, like grills and water purifiers, require planks for fuel. Or planks have to be broken down into smaller bits that get used as fuel.
Injuries and illnesses, which slow the player down or maybe even prevent certain actions until healed.
Sun burn - Over time, the chances of getting sunburn go down, but never completely go away. Slows down the players movement/actions until treated and healed. Healing requires time and rest out of the sun. this makes covered shelter important. Healing time and slowing effect can be reduced by treatment with a healing salve, crafted with fish oil (contains vitamin E) and something else soothing, maybe ground coconut. Maybe an advanced craft would also be a thatch hat to reduce sunburn chance.
Hyperthermia/over-heating - Increases water reduction rate and slows the players actions. Simply requires time spent out of the sun, to cool down.
Fall damage - Just does an amount of damage to go along with the distance fallen and requires time/rest to heal.
Strains/sprains - Can vary in severity and the severity determines if the player is merely slowed in some actions or prevented from doing them at all. Can be helped by wrapping the affected limb with a bandage (crafted with thatch), reducing the severity, but will only heal with time and rest.
Lacerations/bleeding - Damage over time if not treated with a bandage. Varying severity, depending on cause (large from a shark bite, smaller from a crafting/building/gathering accident or fall). Severity determines limitations, similar to a sprain/strain. Heals over a longer time and with rest.
Infection - Goes along with lacerations. % chance to get an infection determined by severity of the laceration, increases daily if not taken care of. Can be reduced/eliminated by cleaning the wound with fresh water and additionally treating with healing salve; higher chances can be reduced multiple times with daily cleaning/treatment. Getting an infection causes a damage over time effect and makes the player sick (reduced movement, reduced effectiveness of eating drinking, etc) until the player fights it off with rest or dies.