Hey, have you heard? Gamepads for smartphones and tablets are a thing. A thing that is quickly becoming the Android equivalent of the many ridiculous iPhone plug-in accessories you'll find all across the web.
So, we're covering another one. It's called PhoneJoy Play. It does this:
You can fit a phone up to the size of a One X in the PhoneJoy Play horizontally, presumably. It connects (hold your applause, please) via Bluetooth, it requires games to implement compatibility (likely through an SDK to get full, native functionality), and you can fold it up into a more compact shape for play on televisions or tablets.