I applaud the idea. I think the case should be flat, considering the goal here, and you can always add feet. Kailh have the new low profile switches (PG1350) with Cherry ML-like caps, and they released new ones, that are even lower profile, which would be interesting too. I'm in the process of ordering some swithces from Kailh myself, to test them out.
60% is a good start, but maybe something even smaller would be nice, to make this keyboard super portable, like those split bar 40% ones or an 60-70 keycap ortho.