I've been looking for something like that too, but the problem with this is the fact that everyone that wants to participate has to be on the same page and that

I've been looking for something like that too, but the problem with this is the fact that everyone that wants to participate has to be on the same page and that includes third party plugins and instruments. Of course you can't assume that the guy you're working with has the same plugins. How could that be handled? A bounced audio track won't allow a lot of deeper editing. Midi only doesn't tell the recipient anything about the instrument you used in your original version. Of course I'm describing the issues from a perspective of someone who tends to rather work with synthesized material than recorded audio, which doesn't apply to every artist, yet it remains a very difficult problem, a dropbox folder or simple file exchange won't make it work in my opinion.