blah, I think the file sharing thing isn't necessary. KaZaA Lite is plenty good enough.
Here's a few ideas:
System Monitor (with a keylogger, clipboard sniffer, URL logger, window logger, all da good stuff) But then STARR by http://www.iopus.com
is plenty good enough.
A desktop controlling utility. There's a bunch out there, but they're usually trojan's which are hunted by norton or huge applications meant for network admins or something. We can make one which is simple and provides basic viewing of another desktop as well as controlling the remote desktop's mouse and keyboard.
We can make a stealth FTP server. There's a lot of trojan's out there, but most require a proprietary client. Also, they have a bunch of features you don't really need. So we can make something which you can use with your favorite FTP client. It can provide remote file, registry, and execute access.
How about a socket script? Maybe somebody isn't a programmer, but they'd like to make little scripts to do stuff like upload/download files and various connectivity stuff.
Then there's a mouse/keyboard macro maker. This can be useful in games and banner clicking while you're away. Shoot, it can be useful for many reasons.
Well there's a few. I never run out of ideas, so if you'd like to consider more, let me know.