STM32F070F6
I don't think that it allows crystal-less USB operation, though. 
Sure fits nicely, though (SSOP20 vs SOL16, eh? Interesting the way that works out.)
flip/pinswap the reset switch so the reset signal doesn't have to snake around so much? Or is that intentional?
I'm a little worried about the uSD card having room to slide out...
Newer Arduinos have simplified the op-amp part to only use a single op-amp, which would theoretically permit use of a smaller package. (OTOH, they're using the send opamp as a buffer for the L led.)