It is great that you are thinking along these lines, and a fine demonstration of trust that you are reaching out to the community for ideas.
My first thought would be that you should be setting your goals, not in terms of what the software does/will do, but in terms of the customer problem you are solving.
Richard Branson (Virgin Group) recently said that as far as he is concerned, the problem of music & video DiSTRIBUTION is practically solved electronically, NAVIGATION is almost solved, but the problem of DISCOVERY (finding new stuff you like) is still almost untouched.
So perhaps you might want to re-phrase your goals:
Our goals:
To help customers with large media libraries find & enjoy the media they want, in any format, in any place, in the highest possible quality.
To do this we will:
Build relationships with a wide variety of content providers & device manufacturers
Use multiple techniques to help customers find music they'll enjoy
Organize, Tag, and Play any media
Connect any device
Serve any file securely
Host any digital media service
Convert formats and DRM's as needed
Offer multiple interfaces and support third party front ends
Simplify whenever possible
If I had to pick the key competitive differentiator out of the above, it would be the connectivity aspect - working with all content providers and device manufacturers. This is tricky, because it is out of your hands - but still worth pursuing.
If I look at what MC actually is right now, and what differentiates it (ie. what makes it worth $$$ vs free alternatives) it is probably the focus on managing large libraries of media, the support for multiple media types and ability to form part of a larger system. This probably means that you can refine your target market segment, to those customers who have large collections of media which they manage themselves.
If electronic distribution & broadband speeds evolve, it is foreseeable that customers would not manage their data themselves, rather stream it on demand. So something like MC would work best in a hosted model.
The focus should be on finding and playing media, not organizing & tagging. The latter are means to an end. I would strongly suggest working with MoodLogic, MusicBrainz etc. to include support for those navigation techniques in MC.
Another thing to think about moving metadata handling one step beyond per-song tagging. Currently MC stores data about individual songs, which it processes to infer attributes of albums. It might be interesting to extend the tagging model to better reflect entities like albums, artists, etc and explicitly hold info about them. This would especially benefit things like classical music.
You might want to consider tying up with Meedio, or a similar product. That way you could leave (some of) the user interface development to them, and focus MC on navigation and organization.
Anyway, just some random thoughts, hope it is helpful.