This is pretty neat for testing gamepad input, but feels rather barebones for such a useful program. So here are some useful suggestions to make this a better experience.
1. Have actual SFX and music; you could even let users choose music from the Audio Portal to suit their preferences.
2. Support specific gamepad features such as touchpads, haptics/rumble, clickable analog sticks, back paddles/buttons, speakers, audio jack, and gyroscope/accelerometer.
3. Build specific tests for other input devices like dance pads, racing wheels, arcade sticks, trackball mice, and VR controllers.
4. Consider accessibility options for people with physical disabilities.
5. Make it work on mobile devices when a gamepad is connected.
6. Make it a keyboard to gamepad mapper for web games that don't have controller support by default.
This gadget is working fairly well for my DS4 controller, but I'm not sure if it's the same for Nintendo or IOS gamepads, so maybe someone else can inform me on this.