It would be nice to hear a bunch of ideas about it, because at this point I haven't given it too much thought.
My thinking is to use something like this, maybe with a separate tiny PCB to connect between both boards because I don't want to mess up the general outline of the board.
The best possible version of this that I can imagine would allow for two boards to mate in all possible directions, you might want to go with a 'slave' expansion board that ties on to the first module in E/S/W but not N, another alternative would be to physically connect them to each other resulting in immediate connections between all boards. Lots of possibilities here. Something like i^c might help connecting the boards on a very cheap bus to allow for the configuration and read-out of slave boards, higher speed interaction would require a free set of 'edges' that can be connected between adjacent boards.
It would be nice to hear a bunch of ideas about it, because at this point I haven't given it too much thought.
My thinking is to use something like this, maybe with a separate tiny PCB to connect between both boards because I don't want to mess up the general outline of the board.