If you have a Wii Classic Controller and a free Arduino Leonardo, with some wires you can turn them to a USB Gamepad within 15 minutes.
I've tried this on both Windows 10 and 'Raspberry Pi's RetroPie gaming machine'.
This guide assumes that you're familiar with Arduino IDE, you've already known how to install Arduino libraries.
So let's start! :)
Step 1: Hardware Connections
First of all we need to connect the Wii Classic Controller to Arduino Leonardo using Two Wires (and 2 more for 3.3V VCC and GND of course).
I used the Wii Nunchuck Adapter for the convenience. You may find it around $1 in many online stores.
Step 2: Upload Sketch to Arduino Leonardo
I use Arduino V1.8.1 https://www.arduino.cc/en/Main/Software
To simulate a USB Joystick, I use the Joystick library from Matthew Heironimus:
And finally, my sketch, will query Wii Controller's state via I2C and map to the corresponding Joystick's buttons.
Pressing the Home button will toggle between XBox controller and Standard Gamepad button mapping (with LED 13 indicator).
I hope this will be useful for someone. Happy gaming! :)