> Alternatively, put the IPV4 block as part of the IPV8. For example, the address 100.101.102.103 in IPV4 could just be 0.0.0.0.100.101.102.103 in a new, expanded 64-bit address space.
There's something very important that you didn't address in your post: how do existing, unmodified v4 clients talk to your new "v8" hosts? I'm guessing you didn't address it because you have no idea how to do it, which is completely understandable because it's impossible.
Other than the impossible bit, your suggestion is basically v6.