Perhaps some sort of shark-repelling vegetation that you can hang in the water to decrease the chance of the shark attacking the surrounding tiles? it wouldnt completely prevent the shark from attacking those tiles, and, the more of them you use, the less effective they are, because the shark gets used to them
also, maybe a system in which food rots if you leave it on an idle cooking tool without picking it up or if you dont store it in a chest. rotten food would decrease health, and only help hunger half as much as its regular counterpart
a tutorial at the beginning could be helpful, along with an option to decide whether or not to play the tutorial for every new save file.
additionally, maybe there could be some random/silly "secret" interactions, which would be hinted at or given by floating bottled messages (e.g. trees are more fruitful when planted with mackerel)
there could also be a "chill mode", where the shark comes less often or not at all, but debris isnt as common and items require more resources to build
a system of item upgrades would be cool. an upgraded fishing rod would catch a greater variety of fish, the hook would go farther, the spear would do more damage, the building hammer would gain the ability to build a greater variety of items, the axe would become more efficient, etc. this could even be applied to appliances; the cooking and purification appliances would become faster, nets would become wider, etc.
animals other than the shark would be amazing. you could build a terrarium for turtles, a tank for fish, etc. the downside of these items could be that the shark would attack more often, and, if a creature falls into the ocean, the shark would come immediately.