Interesting, I hadn’t seen that before. Sort of Type-Matrix-ish. I like the Ergodox keycap compatibility…
With uniform 1.5u side columns you lose that nice slope on the side keys, which is too bad, but maybe not a deal breaker. This is what you get with the straight side columns and no other special key sizes (other than space):
From there the obvious choice seems like extending out Enter and Left Shift. Then I want to extend Right Shift to shift over R4 again, filling things in and adding symmetry. But that leads back where we started… 
Maybe instead of extending outwards to fill the R3 and R4 gaps, Shift and Enter should stay the same size and just shift to the right? But then how to fill that awkward space…
Edit: Ok, last one for the night… This eliminates Menu and actually moves a few pinky keys to different rows (
) but has a nice overall balance to it. Ctrl not being in the very bottom right could be a dealbreaker though (or just having a non-modifier in the modifer row at all…)
Edit 2: Ok, THIS is the last one… Mod keys are bigger again and Ctrl is where you expect it. These last two are only 14u wide, which I kind of like.
