Decide on Pinout

[boredom101]

Not all the pins can fit on the expansion card front, so there has to be a decision on which ones should be removed.

[cognitivegears]

If I can suggest, my priority list would be i2c and/or spi, then others. From that you can always hook up Stemma QT and/or QWIIC devices, and use all the little devices (including IO expansion) from Adafruit / SparkFun / etc. Overall, my priority list would be something like:

1. I2C
2. TTL Serial
3. SPI
4. NeoPixel indicator on outside shell
5. Button OR capacitive touch pad
6. more I/O pins