By definition there are only ~4 billion IPv4 addresses available. No matter how fair you distribute them; that is simply not sufficient for todays -let alone future- demand.
The client can always select to use “server -4 *.pool.ntp.org” to get IPv4 only, or “server -6 *.pool.ntp.org” to get IPv6 only. There is no need to break the geographical structure by adding a special IPv6 subdomain.