I have been doing board bringup for my Onion-Omega based feather. There are some mistakes in the initial revision and I could use more eyes on it. Basic idea is to allow using the pin SOM with Adafruit Feather Ecosystem of daughter boards. As such the I2C, SPI and one UART are passed through to the Feather headers. There is also a charge controller, USB booster for powering 5v USB stuff from a battery and the onion. I added an i2c ADC to serve the feather analog pins. The ethernet is also exposed. I hope this will be useful to others. Please review the design and lodge any issues or help with drivers for subsystems here : https://github.com/whatnick/Onion_Feather
Connectivity to the charger and adc has been verified with i2c bus scan and they are pending drivers in C/python for full utilisation.
The design is also available for review without KiCAD at Cadlab. Folks giving reviews and contributing to improvements will receive some boards from my future revision runs.