As far as applications go, I don't have any ideas that haven't been mentioned. I would, however, love sone kind of viewer application for my Sibelius scores that would allow for a limited amount of editing (transposition, etc.). I'd also love a viewer for my Pyware 3d files that would enable them to animate within the application.
I know I can accomplish some of these things with PDFs and video files, but it would be wonderful to eliminate the middle-man, so to speak.