Two things I hope not to see in this game are:
1. Generic world themes, like "fireland", "iceland", "waterland", etc.
2. Scary worlds. As a kid I really disliked this in games (and I still do). Levels like the ghosthouse in Mario 64 (that piano... brr..), the redeads in Zelda OoT and MM, creepy castle in DK64 (even though great music). It's just something I didn't want to be confronted with as a kid.
I hope to see instead unique themes a kid (and therefore the child in me) would love, like (as you did before) space, a theme park, dinosaurs, or cloud cuckooland. And if you really want to include a darker or underground level, make it more like the Glitter Gulch Mine: Fun theme, jolly music, and lots of lights.