Hi,
i agree with the notion of making it work within the Arduino IDE without needing too many additional libraries.
I've been using the sparkfun shield and the accompanying library, and it provides the basic functionality of sending and receiving a CAN-Message. It gives you the message-ID and the data Bytes of a received message and requires the same for sending one. Additionally you can set the ID-Filters (although this isn't very intuitive ... and should definitely be improved, as it's the best way to take load off an Arduino UNO 
)
I think this should be the basic foundation of the DUE-CAN library. Any additional features can be built on top of that in additional library files/layers, but should be modular (imho).
That would be great! I don't know if its possible with the current pinout, but if you could implement a shield with two CAN-controllers, to interface with two independent CAN-Busses or to act as a gateway, it would be fantastic!
I'm excited about this development and I can't wait to see the first implementations!
... Sam