Well, with minimal tweaks it is possible to get full sized arrow keys even on an existing layout.
It could look something like this (pardon my mad skillz in paint):
Of course we still lack dedicated home/end/pg keys but oh well. The only downside of such a layout is that it betrays our intuition on the position of the right shift key.
Still, such a layout already exists in the wild. For example, Lenovo V310 has similar design. And it took me quite a while to stop hitting up when I actually wanted right shift. But that’s a reasonable price to pay for proper (and much more frequent!) arrow keys.
Edit: On a second thought, technically it’s possible to leave the very same top cover that we already have and tweak only the keyboard part. It would require filling some extra space left from the original long shift key and that would not look as clean and aligned as dedicated cutouts, but it’s still doable. That way it would require keys to be slightly bigger and have placeholder parts much like it’s currently done with Enter and | \ keys that laid out differently depending on the standard, but that occupy the same cutout:
vs
