However, my priorities would be fitting with the Mega dimensions, having the proto area, and compatibility with existing library.
That was exactly my thinking. What I wanted was a board that's like a Mega, an Ethernet shield, and a (small) proto shield all squashed into one as a drop-in 100% compatible replacement.
I'm still interested in ideas that deviate from that though, because there could be other specialised versions to follow if there's enough interest.
Jon