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.
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.