Idea 1: Sakupens Lament
Song: Solace of Oblivion by Helblinde
A dark and foboding level focusing on using bright neon colours as a contrast to the constantly black background. I'd imagine this would have quite a lot of decoration, some covering the player and some of it hanging around in the background.
Idea 2: Zazel
Song: Fight for the Present by Nstryder
A monochromatic level, with focus on unique effects that makes the level harder to play. Minimal decoration. A final battle against whoever or whatever "Zazel" is. So... a bossfight level, in a way.
Idea 3: Anomalous
Song: My Ordinary Life by The Living Tombstone
A layout-style level where the entire aim is to make the level feel as out there and bizarre as possible, with lyrics to the song. A test of effects that are the focus for the level that wouldn't fit anything else.
Final Idea: On The Run
Song: On the Run by ShirobonMusic
A simple black and white level, where the entire level is treated as a mini-game, with a similar difficulty to various "Bullet Hell" video games, such as Touhou. Using the full length of the song, and forcing two players. The background effects are minimal as the focus is the actual gameplay.