The idea sounds similar to the 'boss' idea that's been thrown around a few times. There's some cool thought in it but ultimately it wouldn't fit the Audiosurf gameplay as it stands. It would be cool to have something sweeping through the level with you, but not have it interact with the board at all, just eye candy in the background.
Just as a list to where a 'boss' has been discussed in the past (search is your friend. You're welcome
From January 2008: At last
(Dylan -- the Audiosurf dev -- talking about a boss not working out)
From February 2008: Bosses and enemies?
From January 2009: Audiosurf's 'plot'
(minor talk of a hypothetical boss)
From April 2009: ENEMY BOSS FIGHTS
(special attention to this post by Dylan
which talks about him testing modes with characters interacting with the track and how it didn't add anything to the gameplay -- There's a picture which shows the 'enemies' flying alongside the track)
Hope that helps you understand better. I dropped a quick explanation in that final linked thread about how maybe a boss could interact with the score you're getting, with a bonus for taking it down before the end of the level (effectively replacing the medal system). But that still doesn't have it interact with the track at all and removes some of the abstractness of the game, which is part of its appeal.