I've just sent the first iteration to fab. This board is essentially a breakout for the STM32F411CEU6, a Cortex M4 MCU with 128kb of RAM and 512kB of flash. This MCU was chosen as it can run the same software load as the STM32F411RET6, the MCU that powers the Pyboard Lite. I attempted to retain the essential peripheral breakouts (SPI, I2C, USART) while keeping the board size small. There are provisions for soldering a JST connector to attach a battery easily, as well as a supercapacitor footprint for an RTC backup and low power operations.