WHY:
I want to play action/platformer games on my phone. The straw that broke the camel's back was "Prinny: Can I Really Be the Hero?" - I couldn't beat a boss because my fingers kept missing the intangible touchscreen controls. I want a gamepad that is compact to carry, comfortable to hold, and works with multiple phones.
HOW:
This version of the project uses USB-C to directly connect to the phone, and I plan on using an AVR chip to handle USB communication. Using a physical connection complicates phone compatibility but eliminates the battery and has better upgrade potential (battery bank for the phone, headphone jack, microSD slot, etc) through USB. The shell is 3d printed to adapt to multiple phones. Future versions can involve the community for improvements, support for more phones, and artwork.
INSPIRED BY (R.I.P.):
Xperia Play, OpenPandora., HTC Dream (G1), DS Lite, PSP
It's funny. Around the time you posted this project, I was working on this exact idea. I'm no electrical engineer so the extent of my knowledge (for now) is limited. I used dmadison's ArduinoXInput library on GitHub with a Teensy LC to communicate with my phone via USB-C. It only went as far as the breadboard stage with one button and one analog stick.