I think @ofek is the only one of us who has actually implemented PEP 621 at this point. I would be interested to hear his taken on the final version.
I also think it might make sense to do another Provisional acceptance (or whatever PEP 517 is), but on a shorter time period, so that we can see if there are any points of friction that need clarification or adjustment as we try and implement this (particularly in setuptools, which is always the biggest PITA).