I really like both those ideas of adding in some kind of instant messageing and the ability to browse other people's files. In the good ol days of napster, you could see for example that someone was downloading some of your favourite mp3s. Seeing that, you could then start to chat with the guy who was downloading from you about those tunes (or that artist) and he/she might even recommend some other bands which sound similar which he/she also has. And then you could browse their files and find other cool stuff.

If you want to set this client apart from the others, I think these two features would definatly be the ones to do just that. As a developer myself, I dont really see any reason why this would need to be centralized to make it work. But there is the problem of getting the other gnutella vendors to implement those features too, and also decide on a standard. Or maybe the current gnutella protocal could be extended to incorperate these features.

Peace and love