The past couple of weeks I have been working through my long todo list. While doing so, I have added some more ideas. I believe it will take a month or so to test this many boards. I finished up some of the high voltage designs, which should be safe with a little care. The i2c, SPI, i2s buses for Flex Modules will be consistent.

i2c master (STM32) is laid out clockwise SCL, SDA
i2c slaves clockwise SDA, SCL

SPI is meant to fit point to point. ST has a specific layout which we will follow.
SPI: NSS, SCL, MISO, MOSI (which ST maps i2s with -)
i2s: WS, CK, ext_SD(SDI), SD(SDO)

I'll be working on a few more Flex Modules as time permits, especially better ECG designs. However, the priority will be to get documentation, working demo projects, and firmware together to bring interest to the Flex Module concept.