~ Desert core is not done often.
~ I also feel like medieval is a thing but not too popular.
~ National park maybe? haven't heard anyone talk about it, but I've seen it once and thought it was neat
~ Botanical gardens? same thing: haven't heard much about it, but I've seen it toured once on a super small Youtuber's video
~ Neon!
~ Marine science/biology
~ Avengers/DC/superhero
Swamp theme
Jungle theme
I've seen someone do an island in a certain way with terraforming that make it look like some villagers had boat houses instead of just houses. It was pretty cool. Not sure that is really a theme though.
my island theme is nostalgiacore (just started thinking of layout ideas, so unfortunately i don't have a great perspective of how it will lay out) i never really see childish themes excluding kidcore, since that core mainly focuses on playgrounds and bright colors, while i strive for more of a giant 2000s reference. Stuff like barbie castles and pillow pet custom designs for cushions.