The very popular mini arcade case for your ipad was reversed engineered for the iphone. From conception all the way down to an actual prototype, Ben Heck built a bluetooth gamepad with 8 buttons, a d-pad, usb for pass through charging and it also uses the iphones power instead of using additional batteries.
You can watch his modded video here: iphonecontroller
The best part of his video is to see all the steps he took to make it including the case, the buttons and the circuitry. He’s made many other mods, too, which you can watch on his youtube channel.
Here’s the official iCade controller that will be out this spring.