I think about this every day. The identical layout of the full Apple keybord that comes with a Mac Pro, but a mechanical layout, for Mac, with every key backlit and a nice volume wheel on the side, with Cherry MX Blue's. I personally don't care about it being too big or too heavy.
What's stopping me from building it is that I don't know of any keyboard layout exactly like this (for Mac) to even begin switching switches. The closest I could find is the Das 4 for Mac which I just joined this site to find out if I could add LED's to which would get me pretty close to what I want. The bright ice-blue lit keys that I had on a Ganss GK87 keyboard (bottom photo) would be fantastic.
If anyone happens to know where I could get a mechanical for-Mac board like this to start with, even custom built, I'm very interested to know... I'm sure out of all the other people who spent $5K+ on an iMac Pro there are others besides me who would also pay a pretty penny for the ideal keyboard to go with it.