Here's some ideas for originality:

• Try making custom enemies.
• Add in custom items. This is super easy once you understand trigger zones. Look at foo races for more info.
• Be different. Make up level types. Use tilesets in messed-up ways. Fool around with the tileset and discover some secrets, such as the parallax-able bricks in Tubelectric.
• Make a hotel.

Dang, all this makes me want to make new levels. ;P