Is “IPv10” a joke or a serious RFC draft?

I suspect its a joke ala OSI Layer8. Given that IPv6 adoption has been incredibly slow, more IP standards are unlikely to be transitioned to.

If you're really picky about layouts, you can just reuse your IPv4 address with your IPv6 address (given you have a prefix/etc)

IE: My IP is 1.1.1.1, my IPv6 prefix is 2607:ff58::/32, I can make my IPv6 IP 2607:ff58::1.1.1.1 (aka 2607:ff58::101:101). And if you're especially keen you can do some awful NAT tricks...