looks like it will be an awesome game.
umm.... about the music, i do have some suggestions:
For underwater, use: Metal Slug 3 - Hard Water
If ur in a cave, use: Metal Slug 3 - Devil's Snow Cave
Legendary Pokemon: Children of Bodom - Angels Don't Kill
Gym Leader, use: Children of Bodom - Are you dead yet?
Elite Four, use: Breaking Benjamin - Blow Me Away
Champion, use: Metal Slug 5 - Final Attack
Title, use: Saliva - Ladies and Gentlemen
Wild Battle, use: Metal Slug 7 - Allen's Theme
Ordinary Trainer, use: Saliva - Click Click Boom
Grunt, use: Saliva - Superstar
Gang Leader, use: Metallica - Ecstasy of Boom
Rival, use: Megadeth - Holy Wars
Hope this helps :P
Best regards,
Hank