Well, I've played this game for a few hours and I think I have a good idea about some additions. I made this list with the idea in mind that these can probably pretty easily be implanted in the game's current state, unlike ideas about multiplayer and islands. Also I realized after the fact that this list focusing heavily on thirst and hunger.
1. Potatoes and Beetroots: Don't get me wrong, I love diversity, but as far as I can tell these two are almost exactly the same. The grow for the same amount of time, they provide the same hunger, they're equally rare, etc. So to separate their uses a bit, I was thinking that Potatoes could provide more hunger than Beetroots, but Beetroots provide more thirst. It would give players a reason to pick one over the other in certain situations. Speaking of thirst and vegetables...
2. Vegetables and Thirst: Why do cooked vegetables provide thirst while uncooked vegetables provide none (or close to it)? Shouldn't cooking them actually make them have less water, because of evaporation? At the least, I think uncooked vegetables should give at least some thirst.
3. Hunger and Thirst Death: This is sort of a big one; when your thirst or hunger goes down to zero, you immediately die. I actually had a really nice raft going but neglected my thirst for a while and just died out of nowhere. Thirst and hunger should start to lower your health instead, even in reality you would start to decline in health before death, not just drop dead (like suffocating in water would do).
EDIT: It seems like it does lower your health, except when it starts flashing red, not when its empty. The screen should start turning red or dark when you're close to death, its really easy to just let your health go down that way and not know it.
4. Coconut Seeds: Uh, do you know that coconuts are the seeds of palm trees (at least the coconut producing species, Cocos nucifera)? So really I think the "coconut seeds" should just be replaced with a plant-able coconut. In fact, coconuts should be found floating around. It makes sense, they do float and other parts of trees are in the water, plus this would provide the player with some food and water early on, and with lumber and thatch for later.
5. Coconut Food: Coconuts have an edible inside, usually called the "meat", so shouldn't they also provide some hunger? Granted, they probably should give a lot more thirst than hunger, but still.
EDIT: They do give hunger, I was wrong. Disregard 5.
6. Swimming: The ability to swim underwater would be nice, although I can't think of a particular use right now. It would just be a bit fun, maybe expand on it later with underwater things?
7. Fishing Time: For starters, I think fishing is way too quick. Mackerel is a really good food that can be caught very quickly in a short amount of time. It doesn't need to take a long time, but just a bit longer.
EDIT: Ok, really everything can take a bit longer to do, at least for growing vegetables and trees. The water purifier and cooking spit seem about fine.
8. More Fish: More fish would be great, like pufferfish that can be used to make poison to hurt the shark or rarer fish that give more food. Just some more diversity for fish. Also fish that swim around like the shark that can be caught would be awesome, but a bit harder to do I understand.
9. Shark Fighting: Fighting the shark right now basically consists of waiting for it to attack your raft, then shanking it 3-5 times with the spear, and then either harvesting the meat or watch it swim off. The shark attacking the part of your raft with your Water Purifier early in the game is basically a death sentence because you probably don't have a spear yet, which is terrible if you're unlucky enough. So, I think there needs to be more ways to not only fight off the shark, but also hunt them. First, I think the Axe and Hook should able to deal damage to the shark, but only a small amount, that way people can somewhat defend early in the game. Also, there should be a way to more easily attack the shark without having to wait for it to attack the raft, like throwing the spear or something. Otherwise its just a chore waiting around for it to attack and then shanking it and waiting and shanking and waiting and butchering and waiting all over again. Plus adding more sharks as the game progress would be great, which I don't think it does right now. Someone else suggested that possibly sharks grow in number the more of them there are, meaning you need to keep them in-check or else be swarmed, which is a great idea.
EDIT: Shark damage is low as hell right now, to the point they're not a threat. I can easily kill a shark by just shanking it while in the water while being attacked. Later into the game, I eat almost purely shark meat because its so easy. We really need more sharks and for them to do more damage. So what I said above doesn't fully apply anymore, but some still does. Also maybe the Axe should be used to harvest the shark instead of the Spear.
10. Throwing Coconuts: The ability to throw coconuts at the shark would also be fun and ties in to the above idea of early shark defense. Especially if coconuts are found floating around, as I suggested earlier.
11. Current: It would be nice if the current moved the floating items at a slight angle, not dead on like right now. It just feels artificial. I don't think it would really mess up the nets.
12. Corpses: Alright, this sort of breaks my rule about decently easy/quick things to implant (in this case, the model), but finding a floating corpse would be interesting, especially if you could choose to butcher him like the shark, plus maybe he/she has some rare items or something. An interesting moral decision, something real survivors have to cope with sometimes.
So yeah, my suggestion so far. This game has a lot of potential and I hope it isn't squandered like Stranded Deep's was. Keep those updates coming, cheers!