Another board I started a long time ago, but due to time/life, has been on the shelf.

M7 board is connected between a Z80 and it’s dil socket.
For starters aimed at the Amstrad GX4000 console (and of course the other Amstrads), boosting it to be a full blown CPC+ computer. But maybe later other Z80 based computers.
It gives similar features of M4 board ie.:

• WiFi abilities
• SD card as mass storage
• Rom board
• Cartridge emulation
• and much more.

However due to the design it will be able to offer more than the M4 board.
The Z80 datalines can be intercepted, so technically any hardware can be emulated on the Cortex M7 chip (disc drives, tapes, keyboard), providing the Cortex-M7 can keep up.