I've been thinking we could simply extend the ipv4 address to be 11 bytes by (ab)using the options field. That is, add an option that holds more bytes for the source and destination address, which are to be appended to the address already present in the header.

I am thinking that since an option starts with 2 bytes and everything must be padded to a multiple of 4 bytes, we can add 16 bytes to the packet, which would hold 7 extra address bytes per source and destination, giving us 11 byte addresses. ISPs would be given a bunch of 4-byte toplevel addresses and can generate 7-byte suffixes dynamically for their subscribers, in a way that is almost the same as CGNAT used today but without all the problems that has.

Most routers will only need to be updated to pass along the option and otherwise route as normal, because the top level address is already enough to route the packet to the ISP's routers. Then only at the edge will you need to do extra work to route the packet to the host. Not setting the option would be equivalent to setting it to all 0s, so all existing public hosts will be automatically addressable with the new scheme.

There will of course need to be a lot more work done for DNS, DHCP, syntax in programs, etc, but it would be a much easier and more gradual transition than IPv6 is demanding.