I think you should add the following to make it into an actual controller:
D button (many games use the xbox 360 4-button standard)
Start and Select buttons (pause games, etc)
Dual brown townog sticks (movement)
Switch to turn on/off compass sensor
Guide button? (for things like Steam Big Picture)
Shoulder/Bumper buttons? (May be a bit difficult to do, but maybe just adding 4 buttons to the top would be good).