Do you care about real world ecology? Islands in fiction frequently have the opposite of reality - think monster island with King Kong running around fighting dinosaurs and other giant monsters. Opposite of the real world, but who cares? Besides, dinosaurs!
I'd have fun with giant floating cities that either sail the seas via magic or are towed by giant dragon turtles. So many options with that, are they peaceful traders? Ravaging horde that has ensorcelled the dragon turtles?
What about underwater civilizations? Perhaps lost to time - these could be something that doubles as dungeons. In my own world I have a region that was suddenly and cataclysmically flooded based on the Mediterranean (which was at one point dry land). But islands can sink as well, perhaps because of a long lost war or melting ice caps? In any case, in my scenario the lost cities only become accessible certain times of year and are protected by powerful magic which didn't really work as planned.
Could be fun, maybe more later.
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