You just gave me an idea, aquabluu.
There should be "full-combo" challenges, where you must either FC the song or play on Sudden-Death (same either way), and try to get as high as possible score that way.
The types of songs should be:
1. Songs with forced "speed-up". (Urban Fragments, that 7-star song used in the fire robot boss, Kechadon2000)
2. Gimmick songs. (Ordyne, Mori no Kumasan Ura)
3. Songs with very long/hard strings. (Yawaraka Sensha Ura, Senpuu no Mai, Etude Op. 10-4, Hikari no Kanata e)
4. Weird rhythms/notes. (Blue-chan 2, Stage 0.ac11, that 9-star "Kumonai" thing, Many Wow Bang! Regular + Ura)
5. Slow BPM with 24/32 notes. (Mappy Medley, Family Dondon, Tank! Regular + Ura)
6. 999 combo songs. (Bachi Osensei Ura, Kurenai, Punishment)
7. God Songs. (Rotter Ura, Ryougen no Mai, Kita2000, Hataraku2000, 2000 Series in general)