Suggestions PART 1

SUGGESTIONS PART 2

Houseboats like in the sims 3 would be a fun idea, but you definitely have to add the two most popular parts of island paradise to the Sims 4, the resort, and the underwater diving. EA.... I don't know what made you keep those out of the sims 4, but that was a big mistake.
Vehicles.  and you could include more than just cars. You have types of boats, why not make airplanes a thing? You could give the sims 4 a new way of traveling to worlds, or a new career as a pilot or flight attendant where you could possibly travel with your sim on the plane. Or even adding an airport.
Update paintings.  I have been playing the sims 4 since it came out and the paintings have been the same forever. As well as the photography poses, suggest you add some more poses. 
Add shopping, but not like the flea market. Add shops in the game where your sim can browse, or click on an item, and buy it by carrying it to the register. carrying it to the register would make it seem more realistic and fun, in my opinion. 
Hospitals.  Make them more realistic. First, you could give a sim a wound from something and need to go to the hospital. Then they can sit in the hospital bed and have a nurse and, or doctor take a look. If you take this idea, put in actual effort, and I suggest you raise the salary of the doctor career. Doctors do get paid a lot more. 
CAS.  if you're going to add more to CAS, my suggestion is add more mermaid stuff.
Build Mode.   Also, for future saves, make the furniture, doors, windows, everything match.
Fixes.  remove moola, give sims more room to move. Sometimes sims have a lot of space to get by, but something is just barely in the way and it doesn't allow the sim to move and that gets annoying. What if you make the entire world open instead of one section of a world? Or just take away the loading screen when going to other worlds.