So I'm looking to upgrade from the ATMEGA328PB-AU (B version has 2 I2C ports) to the ATSAMC21G1XA to get more I/Os and more memory. While the SAMD21 seems to have good Arduino support I'm looking to support 5V I/O, hence the SAMC variant.
I'm wondering what kind of pain I'm taking on. Is there a place/project showing what software support there is for the SAMC devices?
SAMC SHOULD be relatively similar to the SAMD, for which there is already a core (many cores, in fact.)
I actually laid out a SAMC board to experiment with (Atmel SAMC breakout | Hackaday.io, but I got a bit disillusioned when I realized that there's no native USB, and that my soldering skill may not be up to QFN 
Mattairtech has a fork of the SAMD core that is modified to work with the SAMC and SAML micros, I haven't personally used it but it looks like it should be ok.