More choices in the retail stores and different stores. They have several high end shops that when I look in there are no guests in there. But the 2 main shops that have the usual fare (t-shirts and keychains) are jammed with guests. Remove some of the high end stores and place regular themed ones in their place.
Disney + in the cabin. I love the TV offerings but would love seeing Disney + offered so that I can try it out before I buy it.
More variety for food on
Castaway Cay. There seems to be chicken, hamburgers, hot dogs and not much else. They have the ability to showcase BBQ on the island it seems to be very lacking.
Behind the scenes of the ships. I love the Art of the Ship tour, but would love even more a behind the scenes of the galley, wheelhouse, stage area and engine spaces. (I know for security you can't , but it would be great to take that tour).
In Animator's Palette I'd love to see a different Crush show than the one that is in regular circulation. Mix it up a bit.
As stated elsewhere different
Castaway Club gifts for different # of cruises. Cruise 1-2 get a bag, cruise 3-4 get a towel, cruise 5-6 get a thermos, etc. I have the same bag and probably will get the same one again next cruise.
A timer on deck chairs. When somebody goes and puts a towel on the chair the timer starts and they get 15 -30 minutes to sit in it. If they are sitting in the chair then the the timer doesn't time and resets when they get up. So like a parking meter. If the meter is Red then the towel comes off and I can sit on the chair. If its green then you're just getting a bite to eat or using the rest room, its occupied. To reset it you have to sit in the chair.
I know I'll think of more, but this is a good first pass.