Of course, but you need to be very careful to get everything right and secure.
I'm sure it's not the client you don't like, but the GUI.
Maybe you can change what you don't like and publish some patches for the official client.
This. If it is GUI related, you can just isolate the core classes and put your GUI code in a seperate file, will make it even easiers for others to adopt it. If you want to change part of the core code, that's another story altogether.