Personally I love the Z88's keyboard, but I can understand the need to have a path avaiable for an alternative replacement. One of my Z88 keyboards has gone soft around the enter and cursor keys which causes accidental key presses more often than I'd like. Getting two presses of 'Enter' when you only wanted one becomes annoying very quickly.
This is for ZX81 and not Z88, could still be useful though if someone like to design for Z88
https://www.zx81keyboardadventure.com/2 ... s-and.html
To be fair, I haven't typed on one personally--my Z88 is still on its way to me--but I have a Toughbook with a rubber keyboard and it's an absolute nightmare to type on, so I assumed similar from the Z88. Sounds like that may not be the case, but I think a mechanical keybed would be a fun DIY project all the same, and more options is always better, right?