Here's a level I made. It took me a long time and tons of tweaking. It actually got to the point where it was on hold for months. I would guess the total amount of time it took to make was in the dozens of hours (not to say it's nearly good enough to warrant it).
Level name: Heaven & Hell
Concept: A level where Mario descends through Hell and then returns to Heaven. Along the way you fight the same enemies, but with different variants (one "Hell" variant and one "Heaven" variant). The hell level is a challenging, scary, platforming experience, whilst the heaven portion is more like a schmup (albeit very basic).
Course ID: 994-V7Y-1SF