Behold! The Level I've been dreaming about making since before I was able to pick this game up!
Lakitu Mineshaft Finale (9806-0000-0076-ECD4)
Style: NSMB Underground
Concept: I should have called it Mines of Mario, as it is epic. This is the final leg of the someday-will-be-concluded journey to the heart of the Lakitu Mining Facility, where they, you know, mine enemies and powerups. Expect tunnels, diverging paths, conveyor belts, incinerators, and parts that might still need a little clearing. Be glad you're wearing your hard-hat!
Difficulty: NSMB World 7 or 8, maybe, or like a Dr. Wily stage from Mega Man 2.