Kinda makes me think, there should be something like one gigantic hub for music that matches stage hacks. With the Custom Sound Engine and the SSS expansion codes it's not like we'd have to sacrifice anything existing and new songs could just be added separately. With 11-song (Halberd) playlists.
Something like this already existed but died:
http://forums.kc-mm.com/index.php?topic=18220 But that was for normal stages, not custom stages. Anyone else think this would be a good idea for custom stages? 'Cause I can try and start something if it is.
Also, to answer your question, it depends on music tastes (hard rock vs. techno and dance), but I personally think that a techno/dance soundtrack would fit best for Robz' Kitty Corp City and (more blatantly) Club, as mariofan described above. And...in that department, I have two shameless self-promotions:
A somewhat upbeat and heavier techno song:
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=15932A heavy, catchy techno remix of an obscure video game song:
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=15838Hope this helps you out as well.