Looks sharp with the white solder mask.
I've done the same with 328s and 1284s with more power & Gnd connections if desired, haven't done a mating USB/Serial adapter yet. Was thinking of doing a power supply module too, but small stuff is really intended to be battery powered, say 3 AA to AAA or a flat 3.7V LiPo.
RS232 module, shift register module, too many ideas to implement. Seems at some point it gets easier & cheaper to spread out vs stackup and not have so many connectors.http://www.crossroadsfencing.com/BobuinoRev17