OldMicroGuy:
Nice But ......
I already own several 5 Volt Pro Minis . Don’t really want to buy a 3.3 Volt, 8 MHz version.
Does the board route Vin to the Pro Mini?
Yes, it does, so you can use either a 5V or 3v3 Pro Mini.
Actually, a possible neat solution would be to use a 5V power supply for Vin, so that you can put 5V onto the 5V pin on the power header without any additional cost. (That's what the VIn/5V jumper header is for. Jumper it if using 5V Vin, disconnect if not.).
Running a 5V Vin on a 5V Pro Mini would will mean it running at a lower Vcc than 5V, though; how much lower depending on what onboard regulator is used on the particular Pro Mini. Just depends on the characteristics of the linear regulator used.
But if it is at or above 3.78V, it will still be in spec to run at 16MHz.
Note that the 328p chips on a 5V Pro Mini and 3v3 Pro Mini are identical. It is only the onboard regulators and crystal frequencies that vary. So there is nothing wrong with running a 328p clocked at 16MHz at, say, 3.8V.
OldMicroGuy:
I'd like to see what you come up with using ATmega1284P.
I was thinking same basic idea with the nRF24L01+ headers etc., but maybe a longer board (about 5x10cm, i.e., Mega/Due size), with the extra room for the chip itself, the bigger additional breakout header, and the balance going to some extra prototyping space. I expect I will end up doing it just to satisfy curiosity, but I'd be quite surprised if turned out to be very popular, as my suspicion is that for the few extra bucks the Teensy 3.x would just represent a much better value proposition all around!
But who knows, it may find a niche. We shall see. 