It is probably too late. But the Arduino mega 2560 form factor would be nice.
Many people(me included) use a RaspberryPi with an Arduino mega2560 and Ramps board for 3d printing. With a meaga2560 form factor and a 3.3V Ramps board this board could be a much better solution for 3d printing. It would be better because it would replace the Arduino and the Raspberry. And it would also have much more performance on the real time part. That would help with complicated movement especially with Delta printers. Octoprint could run in the Linux section, Marlin 2.0 could run on the M4 section.
The 40-pin connector, if RaspberryPi compatible does have less than 40 GPIO and it would not be enough to drive a printer. Therefore as said in a much earlier post. More GPIO would be better !!!
From the performance point of view the most recent RaspberryPis will be much better on CPU power. So the real benefit of this CPU is the integrated Cortex-M4. And that is best for real time control stuff. And therefore the M4 needs as many Pins as possible! (GPIO, SPI, I2C, UART)