Super Mario World is my favorite Mario game (it's my original, so it's the one that all other Marios are compared to.) And I love spooky things (Halloween's my favorite holiday. Oddly enough, I'm really squeamish about gore - but then I do like horror that focuses on weirdness and creepiness rather than just violence, like the Shining.)
Anyway, I've been on a feature-unlocking spree today, starting a new level with each theme only for the next to unlock. In the middle of making my (rather awesome, if I do say so myself) Airship level (that involves hopping from airship to airship,) I unlocked the Ghost House and SMW set/theme.
Included, in addition to the obvious things like Boos (and Boo Buddies) and Dry Bones, are Doors, which you can make and then drag the paired door to elsewhere in the level, and Hidden Blocks, the tool of trolls everywhere.
So I've created a little ghost house that I think lives up to the style of the classic ones. It's full of hidden passages and little deathtraps (my favorite not-exactly-trap is a really long fall that ends on a bunch of quickly-collapsing platforms that you need to climb up the whole height of the level.) There's definitely a theme here that using hidden blocks is always the better choice than using regular ones (I think there's one normal question mark block that doesn't spawn something nasty.)
Philosophically, I always give some hint at what the right choice is. For example, I unlocked sound effects sometime when working on the level, and the first pair of visible blocks pops out a set of Boo Buddies in a crowded space, but if you land on them, you'll get some creepy screams, which should be your indication to leave them alone.
Right now I think I'm mostly just playing with ideas that only evolve a little bit. But this is only the third day I've had the game, so I expect that over time I'll come up with some more interesting things.
It's definitely a tricky little balance to get a level that's not impossibly hard or trivially easy. My general rule is be "tough but fair," and I also have to remind myself that a level that feels easy to me likely does because I'm the one who designed it. I'll need to get friends to try out my levels to see how difficult that they really are.