I really think it'd be good to get some standards down, so new clients could focus on the interface and features more quickly. Creating a standard for the returning of information on audio files (like MP3 ID3 tags) would be very helpful to all developers, especially if basic source code were given out in various languages. Maybe even create some modules for Perl/C/Java etc. so people can have some ready-written code to integreate into their client for various features?

It'd also be nice to see some cross-client work on developing the network, and the protocol, so that all the clients can integreate new network features easily and strengthen the network. Problems I see currently are the lack of superpeers, and the lack of a decent resume function., which could be aided by more work on the protocol, perhaps with more detail being sent out by superpeers than a list of IPs? I don't know much about the protocol, other than that it's a three step HTTP-style talk between two peers, so that might be wrong, or silly, or impossible. But I am sure the network itself could be worked on more.