One of my ideologies when coming up with game world's is: Okay, so how can you turn something that should be Happy in 1 game, to something more dark. An example would be the Jungle world (that I haven't posted anything of yet). It would be filled with mutated animals made in one of the Evil computer's freaky experiments. Some, however I would do the reverse, for example the Volcano world (that I also haven't posted anything of). Some levels would have you exploring (you guessed it) a volcano, but others would have you in a peaceful forest in the middle of fall.

That's just my Ideology when designing worlds. I just like to sort-of subvert expectations when making them.

ITS ACID