There should be a processor board with two line 90deg male pin connector for a flat cable.
So, design one. Get it made. Let us know when you have something to sell.
Make SURE that you don't make any decisions that someone else will, years later, question the wisdom of.
The board should also be of smaller dimensions.
I'm expecting yours to be no more than 1cm by 1cm. And, to have Ethernet and USB connections. And to accept all shields currently available.
And you can stack them any way you want.
Use your brain. You can't stack an XBee shield, a GPS shield, and a LCD shield, in any order, and make something useful.