I think a wider lens would be the most natural fit; there is of course the GR21 as a precedent, and the shooting style of the GR lends itself well to wide angles - since it's not usually held up to the eye, it's easy to find creative angles and play with perspective in ways that are most effective with wide angles. That said, a long lens option would be cool... but I could also do with a larger aperture. That might be totally impossible within the constraints of the size, of course, but the GRIIIX has given me a taste of a bit more DoF control inside of a pocket sized camera, if they could squeeze in an f2 it would differentiate itself even more from other compact camera options.
The 28 and 40 make a decent pairing, but a 21 and a 40, combined with the usable crops Ricoh includes, could do 99% of all the photography that I might be likely to want to do.
However, and this is a major caveat, I don't like carrying both GRIII bodies. It's awkward, you never know which is which without checking, and you feel like you're going to drop them when juggling between them. It's a thing which seems like it should work in theory, but in practice falls quite short of the ideal.