First off, I'm loving the game, the latest storage update has been really beneficial.
I think some cool additions could be linked to a physics engine, perhaps your raft needs to be realistically structured else it'll fall apart.
To combat that, buoyancy aids would be needed to keep the raft afloat. The pillars already implemented seem to suggest that structure is important.
Cosmetic items are interesting, perhaps doors and different types of windows.
More creatures to ruin your day, maybe it spawns 2 sharks from time to time?
There's a lot of suggestions that people have already said that I'd love too.