I've been looking for a project that lets me deep-dive on the USB protocol. I finally settled on trying to take wired devices (mice, keyboards, and joysticks) and expose them over bluetooth. Some of the best mechanical keyboards are USB-only, so i thought it would be useful if you could have a device you connect to those devices that exposed them as a wireless bluetooth device. This project contains the end result of my experiments: how to connect a MAX3421E USB Host Controller to a NRF51422 Bluetooth SoC over SPI to proxy basically any USB HID devices over bluetooth.