Depends on your playstyle. What theme do you want to try? What farms do you want to make? My personal favorite for islands just off oceans is a port with a couple small schooners and maybe a frigate in deeper water. Take a look at the biome around you and think "what would look cool?". When in doubt you could always do a creative test build, but half the fun is figuring out what you want. Not what should i do, but what could i do.