Then, if you own an IPv4 address, you effectively own an IPv6 subset. Then we reserve a whole bunch of IPv4 addresses for IPv6-only allocations.
I obviously haven't thought it through in detail, but wouldn't something like that effectively transparently work via IPv4 core infrastructure provided the networks at either end support IPv6 if they're using it? We'd still need NAT for IPv6-only endpoints that need to talk to IPv4-only endpoints. It also wouldn't be anywhere near as clean as IPv6 and would lack a few of the nice features, but... an awesome protocol I can't actually use isn't much use to me.