It's not priority for next features.
As we know every IPv4 have equivalence IPv6 /64 and every IPv6 is /128. So, every IPv4 have 2^64 = 18,446,744,073,709,551,616 IPv6.
The question is why we need 'external' IPv6 if every IPv4 have 'built-in' 18,446,744,073,709,551,616 IPv6?