There are a number of proposals that build on these ideas, for example EnhancedIP. Typically, they would encode the extended addressing info in an options field. They were to late the party and missed the circa 2000-2005 window where it became clear Ipv6 will be an uphill struggle. Back in the day, only naive proposals like ipv7 existed, "ipv4 with larger address" but that solves little.
The fundamental difference compared to dual stack is that there is no technical cost to deploy it outside software updates and very little risk - aside from the dubious security benefits of NAT. Not only the core, but virtually all hardware and software on the Internet today support ipv6, but there is an enormous cost to configure it to work.
This flexibility and drop in upgradability of things like EnhancedIP comes with the significant cost of breaking the internet into 2^32 independent routing domains that prevent you for getting the full benefits of the larger address format. This is an acceptable trade off only in retrospect, after the v6 "failure".
The fundamental difference compared to dual stack is that there is no technical cost to deploy it outside software updates and very little risk - aside from the dubious security benefits of NAT. Not only the core, but virtually all hardware and software on the Internet today support ipv6, but there is an enormous cost to configure it to work.
This flexibility and drop in upgradability of things like EnhancedIP comes with the significant cost of breaking the internet into 2^32 independent routing domains that prevent you for getting the full benefits of the larger address format. This is an acceptable trade off only in retrospect, after the v6 "failure".