Comment by welterde

This is already a thing in IPv6 pretty much. You can write applications IPv6-only and support IPv4 via IPv4-mapped addresses (::ffff:1.2.3.4 for the IPv4 1.2.3.4). The host still needs to be dualstacked for that to work though. In case the host is IPv6-only you can use NAT64 (or similar technologies), where the IPv4-space is embedded behind some other prefix, but the application just talks plain IPv6 and doesn't have to care too much what happens in the background.