I'm curious to know the potential difficulty behind porting Arduino to run on Atmel's SAMC family of processors. I have never attempted to port this level of code before.
Supposedly the ATSAMC21G18A is pin for pin / bit for bit identical to the ATSAMD21G18A used on the Zero except for the fact that it's 5V part. I have to use the SAMC processor since we're designing into a 5V based system but I'd still to use Arduino as our IDE.
very interesting question, I am interested as well ...
But I think the internal architecture and peripheral is a little different --> see attached picture
e.g. where is the USB-Interface on the SAMC ?
SAMC has CAN-Bus / SAMD has I²S / different Set (and Speed) of ADCs / more Analog Comparators / ...
You are correct that they are some internal differences... I saw that there was no USB port as well which makes me question how is it a direct replacement for the SAMD. It's hard to tell if this is final chip design since the SAMC still has not been released to production.
The SAMC Xplained board from Atmel is readily available for purchase and interestingly includes Arduino compliant headers. I guess it's possible this is a sign that support for the SAMC on Arduino may be in the works.
It's surely not a direct replacement for the SAMD.
It's hard to tell if this is final chip design since the SAMC still has not been released to production.
It is produced already -> you can find it on the SAMC Xplained board (which is sold already) 
I guess it's possible this is a sign that support for the SAMC on Arduino may be in the works.
to be honest: I don't think so (time will tell)
another question:
can this MCU run on both 5V and 3.3V ?
Yes it can as it's capable of 2.5 to 5.5V. If you look at the diagrams for the Xplained board they have a very nice arrangement of level converters and transistors allowing you to use select the MCU between 5V and 3.3V while safely keeping the EDBG at it's required 3.3V.
When I said production I meant Atmel making them available for purchase at the distributors. The Xplained boards are using the engineering samples if I am to believe their sales engineers.
