One of the reasons I went back to the F/AT was to get the left-side function keys back. The text editor I've used since the 1980s is driven by the function keys, and the top-row keys on a 101-105 key board are an awkward reach for me.
I did some extensive remapping of the F/AT with xmodmap. I scavenged keys from Model Ms, a DisplayWriter, got some keys from modelfkeyboards.com, and had Unicomp make up a bunch of custom keycaps. I finally had what I wanted... and then I had another brainstorm this morning.
The F/AT has extra barrels under some of the larger keys, so you can split them. Right Shift, Left Shift, 0/Insert, and Enter (BAE) are split. At this moment I haven't taken my keyboard apart to add the springs and flippers, so I just have the new keycaps stuck on the barrels. I have the parts, but I haven't done the M spacebar mod yet either.
As I said, that editor used the function keys extensively, but I'm in the process of replacing it with something else, and not much uses function keys any more. Which means the function keys can largely be repurposed.
[sigh]
I have large hands, and arthritis. The left control key means cramping my hand up awkwardly for the letter keys nearest it. If I put an extra vertical 2u key in the function block, I could just use my little finger for control-near keys and the old key for control-far keys. A "wide" layout for the left-side modifiers, sort of.
So, this is my current layout, with the function block remapped.
Adding a shift key to the left will help, with the cramped-hand thing. There are two columns of punctuation characters on the right; my right hand has plenty of room.
Moving the left alt key over to the function block will make it easier to hit, and the Hyper key can go into the "keyboard ghetto" the left alt key used to reside in. I haven't done anything with Hyper yet.
The top-row number keys get the usual Fn modifier. There are left and right Fn keys on the top row; either side can be used one-handed. Unicomp won't do 3-line keys, so they're just marked for clarity on the diagram.
The split Shifts get Win keys, labeled "Tux".
The old Ctrl key to the left becomes a second Backspace; I stole that from the Colemak guys. It's easier to hit that key with my thumb than to use a finger on Control.
The split Enter gets a 1.5u Backspace above it. The old 1u Backspace becomes a "Menu" key.
F1 is mapped to Esc. I like an upper-left Esc.
Rename, Reload, and Menu Bar are F2, F5, and F10. I use them a lot, and didn't want to have to add an Fn keystroke ahead of them. KDE isn't consistent in how it treated "F10" and "Menu".
Macro is for keyboard macro stuff, when I get to that.
PANIC is just a "Help" key. Eventually it will bring up the folding editor I use for note-taking.