If I ever make the hop to building a fully custom keyboard I'm going all out with f13-f24, add some repeats of a-f around the number pad to allow fast hex input + ":" for IPv6 input (and put an equals key somewhere in there!), put the numberpad on the left so it's a bit more balanced, move num/caps/scroll lock all up by each other in the corner as small buttons instead of in the body of the keyboard as various sized ones, turn the normal caps lock into a firmware level layer key, and add a key for shortcutting unicode inputs to the right of the 2 function rows.

This was the initial layout I came up with, there are a few things I'd probably change on the hardware layout side but as far as actual keys I usually go with blank caps so that can be changed without much pain so long as it isn't a special shaped key: https://i.imgur.com/zVJWbC9.png