I'm now working on a GUI for the C64FC. Yes, it has now reached the point where I can actually play with some cosmetics. I'm using Qt, LibUSB and QHexEdit2 for this project, which means you'll be able to run the application on Linux, Mac OS X and Windows without too much hazzle.
The application is quite basic, but shows the abilities the cartridge has, such as running PRG files (also greater than 8 kilobytes), as well as modifying cartridges while they're running!
I personally think this screams for hacks from developers out there. You can easily make a PC controlled SID player (or composer), Koala viewer, video player, program slideshow, or simply use the C64 as a secondary monitor. With a little work you can also transfer keyboard and mouse data as well. The only downside is that it's read-only for the C64, but I'm working on that, too.