Add something among the lines of a research station used to discover more advanced items such as reinforced steel foundations and upgraded versions of existing items.
Add a shark attacking sound effect and make shark attacks far more frequent to balance game.
Multiplayer would obviously be awesome but I would expect a feature like that to at least be in the far, far future.
Some type of ship wreck that spawns rarely where you can discover new "modern" recipes to craft using the research station to firstly find the recipe that you use to craft it.
It would be cool if on the main menu the raft in the background was your actual raft on which ever saved game you are selecting.
A quick transfer button which innstantly moves items from hot bar to inventory and vice versa as well as from chest to inventory and vice versa would be awesome.
A day night and weather system.
Also please nerf the fishing and potato growing systems because getting food right now is far too easy as well as make tools wear down needing you to build new ones.