Perhaps the best way to imagine a theme for that specific overworld segment would be to play a few levels from each world
and see the general environment. You can then try to project it the best you can onto your overworld entry. Note in that post you can also see the theme of the level so that would also help especially if you are not willing to play some levels. I do not except the overworld to be perfect but you should try to the best level you can in consistency and design.