Personally when I am deciding on a theme for my island/town I start with a season. So for example, in NH I wanted my town's DA to be saved during the summer so I initially thought of having a "bold" color pallet. Then since I like the sunset, I decided to go with "bold sunset colors" such as deep pink, purple, blue, and orange. I then built my theme around that, and currently have a tropical island theme that is half resort and half natural jungle. In New Leaf, I went with a spring theme with pastel colors, so lots of pink and white. The path I used also had little hearts on it, so it was definitely more "cutesy."
Anyways, maybe try thinking about seasons first and go from there. As for a basic list of themes, here are some general ones that come into mind (besides cottagecore):
- tropical island
- witchy fall island
- overgrown island (any season)
- kiddie island (primary colors themed (red, blue, yellow) using lots of toys, etc).
- japanese/East Asian island
- european island
- city or town island (with the road path)
- historical islands (ive seen everything from historical England to historical/indigenous Puerto Rico)
- natural park island (very minimalist and natural yet carefully planned out)
- some sort of resort island (could be tropical resort or skiing resort)