I am assuming you have rolled in the ato circuits inside the hat, thats very nice

, gonna save folks little bit of money.
How the relays are connected to the board ? i.e. whats the connector type? It might be worth mounting the connector directly or think of how it upfront. I would love to see us settling on ADJ SR 2 power bars, which uses 12v , db9 connectors.
I think most of us will be just fine to have the controller little bit bigger (like 4"x7"), so throwing some extra space is fine. That will also openup more space for connectors. l293d uses the GND pins as heat sink as well, so the a bigger board will automatically act as beefier heat sink. I am wondering if its better to not design this as hat (due to space constrain, and stacking things together leads to strange pillar like form factor) and instead think of the board as a bigger, flatter pcb and users can solder through hole components in it, and attach the pi header socket and directly mount the pi there. Put otherway, is there any obvious gains by sticking to Pi hat form factor?
This is super duper awesome :0-) . Thank you so much.
If your design works out, then I am game for buying a bantamtools (personal pcb milling machine) and try things myself.