Here is a small list of suggestions I thought of
* Harpoon Guns
* Leviathans/krakens -- generally more enemies
* Scrap Engine (so sharks can't get you but higher tier enemies still can ^)
* More furniture
* Usable furniture for added realism
* More Weapons to build
* Weapon Station to build the new weapons
* Buildable Doors/roofs
* Weather
* Natural Disasters (tsunamis and Whirlpools)
* Pirates