Hello OP, can you do something about the three identical \| R1 R3 R4 keys that ISO users find in their set? You could at least focus on losely approximating and existing ISO layout (with the keys already in your set, without adding anything)... thanks
I already replied to you in this regard. The filler keys are fine. You will find such fillers in most keysets. Btw. R1 pipe key is for HHKB alike layout and not for ISO.
This isn't constructive thinking 
And about the HHKB layout... if someone makes it in ISO, don't you end up with three identical keys? 
Not really. An "ISO HHKB-like layout" would have a vertical ("ISO") Enter key, which would take the place of the R2 1.5U key that's used for Backspace in an HHKB layout, forcing the usage of a 1U Backspace key in R1 (and the other space would be filled by the
`~ key).
Long story short, the
\| in US English layouts gets moved around A LOT and can be found in each of the FIVE (not four) rows in the alpha block. Let's go one by one:
US English over ANSI: horizontal Enter and
R2 1.5 \| key. This is what you'll find on "normal" US keyboards.
US English over HHKB-style: horizontal (ANSI) Enter remains untouched, while the
R2 1.5 key becomes a backspace; the 2U R1 Backspace key is split into two 1U alphas:
\| (moved over from R2) and
`~ (moved over the top left corner of the alpha block, where Esc has been placed into).
US English over ANSISO: horizontal Enter key and
R3 1.0 \| key.
R2 1.5 \| and R3 1.0 \| are direct replacements of each other, and some PCBs are smart enough to map them the same way, detecting which spot has an actual switch soldered into. Note that in an ANSISO keyboard, the left Shift key is still 2.25U long (meaning there is no key between it and Z).
US English over ISO: horizontal Enter key and
R3 1.0 \| key
AND an R4 1.0 \| key between (1.25U) left Shift and Z. There is no actual spec mandating what alphas should go to this third key — instead, there are two different
traditions about it: PC style puts
\| there, while terminal style puts
<> there. Both approaches are equally valid and both have advantanges and disadvantages (note that Microsoft goes with the former; also, the
R4 1.0 \| key is mandatory for UK English).
US English over BAE: EEEEEEEEEEWWWWWW. Big Ass Enter takes the place of both the Enter and the
\| keys (whether you start off ISO or ANSI), forcing the movement of
\| elsewhere. Some keyboards split the 2U Backspace key into two 1U keys:
R1 1 \| and R1 1U Backspace. Others split the
right Shift key on the right side, placing the
\| key in the spot left behind, and even other old keyboards move this alpha to the bottom row, between RALT and RCTRL (this from a time before the Windows keys showed up).
So, after all this, a "good" base kit needs the
\| key present at least three times: 1.0U R1, 1.5U R2 and 1.0U R3; if going with the PC style for ISO, R4 1.0U as well
(applicable to two of the three BAE styles, too).
Hope this helps!
P.S.: I'm purposefully ignoring some exotic physical layouts that would require an R2 1.0U \| key (14.5U keyboards) and a repeated R4 1.0U \| key (ISBAE hybrids). Yay!
