I see three keys that would be nice to have: a 1u Fn, a second 2.25u Shift for 66-key layouts, and a 2u spacebar for Alice and various other boards. The second 1.25u Fn key you could either remove or change to 1.5u. It would be nice to see 4-key ISO coverage as well, but I'm not sure if that's one of your goals for this set.
What's 4-key ISO? I know I've got uk-iso in the numpad kit but unsure about that one.
Everything else you said has been fixed though ( just nuked the second 1.25u fn )
That means supporting the English (USA) national layout over ISO in the base kit (usually shortened to "US ISO"). To this effect, you may add to the base kit the following keys: 1.25U Shift key, vertical ("ISO") Enter key, 1U
\| and either a second
\| key ("PC style") or a
<> key ("terminal style"). The "UK ISO" support can then be kept apart, with three fewer keys.