Good news, the 4 CPUs version of this board is also ready. Now it supports Z80, 65C02, 6809 and 65C816 !
Gerber files are here !
The schematic changes are only at "MCU" section, and this schematic shows below.

By the way, the last parts for the board arrived, so here are some photos.

If you look carrefully, there are some minor errors (mostly at silkscreen), that (all of them) have been corrected at the Z80_SBC.rar gerber files I posted.
1) At silkscreen layer the text about RAM and ROM says "40pin", but ...

Of cource, I designed some Debounce+Single-pulse logic in the CPLD, that is between the Step switch and the Clock input of the Z80. The CPLD design uses the oscillator of the pcb to measure a 20 ms delay period of no-change, and it outputs a clean single clock pulse to the Z80, for single step mode ...

Also, inspired from a video named "Single Stepping a Z80 on a Breadboard" in a youtube channel named "Kenneth Finnegan", I made this adapter for the Z80, so I can see every signal of this processor, when using the Single-step function. It uses 74HC245 buffers.

Yes, you are absolutely right! But...
what I meant, is that my biggest concern is the pcb space, combined with the need for as little as possible via usage (for signal integrity purposes).
Of cource, I know that even 10 MHz is not a problem (the frequency is too low), but... ok.
I promise to design ...

Thank you Bill,
the board is already 10 x 10 cm (which is the upper limit for a low cost pcb manufactured at some pcb houses), so one solution would be to use smd 7-segment displays and smd SD card socket at the other layer, underneath the 7-seg displays.
Adding RTS signal is no problem. T7 or T8 or ...

The schematics are here. I tried to make as little as possible changes to v1.

Changes from your v1 are:
1) 28-pin (32 KB) and 32-pin (512 KB) ICs are supported for both RAM and Program Memory (PROM, UV EPROM and Flash memory). The 28-pin EEPROM is not supported (where, pin 1 = A14 , and pin 27 ...

Thanks Bill for this great design. That inspired me to draw a slightly different layout. I like to see it more like an Z80 (and 6502 / 6809) SBC rather than a CPLD trainer, so with this in mind, I made this (based on your work).
Gerber files included.