There already is a subnet. Actually a few. ::ffff:192.168.0.1 is reserved for when you need to pass an ipv4 address through an ipv6 API (e.g. v6 client application on a dual-stack OS on a dual-stack network) and 64:ff9b::192.168.0.1 is reserved as an actual address for NAT64 gateways to use (v6-only client, network has a NAT64 gateway somewhere on it)