First, this is something I've been looking for for a while.

Is the crystal necessary? I ask because the ItsyBitsyM4 doesn't have one. Also does the VDDCore connect to anything?

This is a great idea for those that want to use all the I/Os on the chip. The footprint  is similar if not identical to the samd21 depending on the suffix and the PCB might be interchangeable. Leaving 5 or 15 pins unavailable as a dev platform should be a crime.  Its hard enough to find a full 8 pin contiguous port for I/O that isn't interlaced with a vital serial, crystal or other peripheral  I/O right in the middle of it. When you limit a chip with this capability on port pins, you aren't doing it any favors. If Microchip would get on the ball and get an Xplained board out for $30, I'd buy it.