I have assembled the hardware. It really looks nice and the step down converters work perfectly (5v & 3,3V). I have tested the M0 bootloader and the PCB enumerates :). The PCB is compatible with the other Arduino M0 boards. I need someone that helps me with modifying the variant.cpp, pins_arduino.h, variant.h, etc. You will receive a board from me to adapt the arduino software.