My attachment for making USB game controllers

I've designed a board which shows up as a USB game controller, or a USB mouse on a PC. I'm considering using it to make a shield which would let Arduino users output analog values and button presses as a mouse, joystick or game pad so they can make their own devices which will work without modification in games and other programs. For example you could read two pots and output the values to this device as the mouse X and Y, and run any computer like an etech-a-sketch, you could read an accelerometer and make a flying mouse, or a joystick which you could use in game by leaning your body, really anything which involves making your own input mouse or game controller device.

I would like to get feedback on a few aspects of the design. First, how would you like to output your values to the board? I can make it work with any method of output the arduino can handle. I'm considering just making it read analog write commands, because then the arduino developer will not need any library to use it, and they have all of the analog pins to choose from. If it is wanted, I could give it an analog input as well, so that pots and other analog signals can be wired to it directly.

Another aspect is weather is should be a shield or a dongle. I don't like that making it a shield locks which pins may be used, but if it has to be wired that adds some complexity to the end user's setup.

And of course I would also like to gauge level of interest.

I don't want to cast doubt on things but...

The Arduino Leonardo, Esplora, Mini, Due, and clone boards based on their designs already allow the existing USB port to emulate a mouse/keyboard. Example code exists in the Arduino IDE. While some may want to add this capability to an Uno or other board, the market would probably be low as it would have to be much cheaper than the above solutions.