This is a rough sketch of the layout that I had in mind originally. It is for PCB-mounted Cherry MX switches.
The builder cuts it in half somewhere in the middle to get whatever key groups he likes on each side. The keys in the middle would then belong to either the left or the right side depending on where the cut was made, or not be part of the keyboard at all.
Each module has been outlined and given a colour, but I have not yet indicated where the traces between modules would be. If such traces are cut off, they could be replaced with a ribbon cable.
I have put 1/2 key spacing between key groups because that is very common among cheap and vintage keyboards whose cases could be reused as building material. If the spacing is different, then the modules could be cut off and connected via ribbon cables.
If you put a num pad on the left side and want 2× high Enter and + keys, then the numpad is left-handed.
I am not at all sure about the bottom row. Personally, I prefer 1.5 Ctrl/Alt, 1.0 Win/Menu and 2.0 wide Space bars, but I think that it would be easier to source new key caps for 1.25 wide modifiers.
The big question is, which split rubber dome keyboards do we want to be able to convert?
I have had the MS Ergo 4000 in mind, but I don't have any on hand to test with.
I see no reason to convert the Goldtouch because its rubber dome switches are quite good to begin with.
.. and also.. should we settle with staggered QWERTY, or should we go with symmetric stagger?
The biggest problem with symmetric stagger is finding key caps that match and still fit into an existing keyboard case.
Edit: Corrected a few colours and symbols that were wrong.