I’ve started building custom arcade controls for using with classic arcade game emulators. All Open Source and Open Hardware of course, with the source code up on GitHub.
STM32F3-Discovery-Arcade is a prototype USB HID device for arcade controls. It currently supports GPIO joysticks and buttons, quadrature trackballs/spinners, and will drive up to 4 channels of NeoPixel RGB LED strings. The project has both custom STM32 firmware and a custom adaptor PCB designed with KiCad.
Please go take a look.