Just seen the on/off shim and it looks really nice! A great little idea!
But a problem I’ve had a lot is that many HATs block the use of the TTY pins, which I tend to use for downloading binaries to my Pi, and general debugging. It occurred to me that the problem is that you can’t have a shim sticking off the side of the pi with extra pins because it wouldn’t fit in any case. But if you had a case that was designed to accomodate a debugging shim off the side, then the sky’s the limit.
So what crossed my mind was a little board like a cross between a Black Hat and the on/off shim, that you push down over the gpio pins and solder on (or ideally don’t solder). With a custom case to house it and the Pi. Then you can add a HAT on top, and the case accounts for the extra height caused by the board so the HAT still rests on top. The board would have a duplicate header outside the confines of the Pi (a 1:1 mapping of pins), for spying on pin states or connecting unused pins to a breadboard. (Or connecting to a black hat.) A little built-in on/off button and power supply would round it out nicely.
There’s scope for using the pins to add little side-HATs, like one with buttons, switches and leds for testing, to save you having to attach it to a breadboard.
Just a thought. I don’t know how useful anyone else would find it.