Mine would be a standard ISO 60%, just like the one I have on the way, only the 6.25 spacebar would be split into four keys: 1+2+2.25+1.
I have noticed I always use my left thumb for space, so it makes sense for me to split the spacebar in two and use the right half for Fn. But then it would be very hard to find 3u or 3.25u caps.
However, 1u spares are easy to find, the 2u is the numpad 0, and the 2.25u is the ANSI left shift. So this layout can be covered with any full ANSI keycap set + ISO kit (or the other way around).
I would want a hole between the G, H and B for a Trackpoint as well, but that's easy enough to drill at home.