Okay, so...Most of those Ideas aren't going to work.
How about this?
1 - Cultural/Ethenic: Entries must be themed around a specific culture or Ethenic group. For example, someone could choose Japanese and would have heaps of Taekos (Japanese drums, I think thats how you spell it) and a Zen-style harp. And african theme would feature bongos and other ethenic drums. An Aboriganie (Australian Natives) would have a digeredoo in theres. Basically, you choose a specific native group of people, and incorperate their musical flavours and techniques in with your own. The winner would be the one who has the entry that best incorperates the selected Culture, with their own flair and style.
2 - Emotional: This a good one! The hosts (Maestro and Co.) would select an emotion, such as love, anger, depression etc. And people have to create a submission that best personifies that emotion, which would allow for a range of genres and styles to be used. Alternativley, you could set a number of emotions, and choose the best of each emotion. The way to judge this one, would be to see how listening to each submission affects your mood. The closer to the theme, the better!
3 - Hypervelocity: The idea of this one, would be to illustrate through sound, an eventuall increase in speed. The trick however, is that you would NOT be allowed to change the temo of your song at any time. This would test the creative skills of artists, as they would be required to start the song off 'slowly' and then increase the 'speed' so that by the end of the song, the song seems 'faster'. Would be a great way to provide songs for future parodies of the upcoming "Speed Racer" movie.
4 - Electronic March: This is an idea I had a long time ago, but have never managed to make a proper song. The basic idea is to make a millitary march song, but in a techno/electronic style. You could also make it a rock style for people who like that kind of thing. But yea, would have a snare-drum line, and some trumpets etc. but with a bassline and synths. Cool huh?
5 - Newgrounds Theme: This would be super popular, or super stupid. But you make a song that personifies Newgrounds, or a specific are of Newgrounds, such as the forums, or portals. Could be good to be used in future flashes, and Newgrounds promotions.
6 - Environment/: Choose an environment, and make a song themed around it. You could have a nice mellow sound for a beach, or a crazy hard-rock sound for a volcanic area. This is probably the best idea I've had thus far, and probably has the most potential. Judges would be looking for the entry that best personifies there chosen environment. A couple could be: Sky, Space, Cave, Jungle, Desert, Snow, City, Underwater, Volcano, Swamp, etc.
Okay, there's some more ideas for you Maestro!