I have an idea I made a thread about a while back, but nobody posted on it.
The ability for Brawl to detect other games saved on the system and use data, namely, Sound Files, from those games. That way, music from a game you have for your Wii could be used in certain stages or a created stage.
It's similar to what the Budokai series does retroactively with it's games. It's just a step above, say, what Psycho Mantis did in Metal Gear Solid, but instead of simply detecting the certain save files and saying their names
The only slight setback is that one would have to program the games that the sound is recieved so that the file is both saved to disc and compatable, a few more lines of copy/pasted code per game. However, if games made previously were not made with such an option in mind, those games could not be used for music, unless they were to rerelease them (not a big possibility).
However, newer games, like, hopefully, Super Mario Galaxy, could be made with such an option in mind, as Brawl was being made simultaneously.