Given that you don't want to tie next-generation allocations to existing v4 prefixes, that scheme ends up more complicated than what we got with v6. You would s

Given that you don't want to tie next-generation allocations to existing v4 prefixes, that scheme ends up more complicated than what we got with v6. You would still need a gateway to keep state if you're moving traffic between v4-only endpoints and v6-only, which would end up pretty much like NAT64. The opportunity to break backwards compatibility in v6 brought many other good things as well: fixed fields that are easier to route, more sane network allocation schemes, router discovery for LANs, and lots of other bits and pieces.