It might be interesting to produce a surface-mount variant of the board. People could then practice their SMT soldering, or get a fab-house to load the boards on a Juki. 0805 would be a reasonable comprise on hand-solderability and board space. You could get a SMT LDO, AtMega, MCP23S17, MCP7940N, MAX3232 along with all the decoupling caps and resistors. That would just leave the headers and connectors to be hand soldered.