We could presumably have done something like: use the IPv4 packet format, treat the 32 bit src/dst address in the header as the first 32 bits of the address and put the remaining 96 bits (+ checksums/etc.) as the first few bytes of the payload. Then create TCPv6, UDPv6, IGMPv6 etc. protocol identifiers for the protocol field to distinguish traffic that's encoding an IPv6 address in the first few bytes of the payload.

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.