Proton VPN supports IPv6 now, and with manual configuration you can get full IPv6 from client to VPN to internet.

With NAT64 supported the user can complete the migration to IPv6 by disabling IPv4 on the client device.

The user would still be able to access IPv4-only services using a DNS64 server that converts IPv4 addresses to IPv6 addresses with the prefix 64:ff9b::. The VPN server would then translate the IPv6 addresses to IPv4. For example 64:ff9b::b077:c880 is the NAT64 version of 176.119.200.128 (mail.protonmail.ch), however 64:ff9b::b077:c880 isn't currently pingable.

If Proton VPN supported NAT64 then the outgoing address could simply be the IPv4 address of the server.

NAT64 could be silently added - the user is required to manually change their DNS server to a DNS64 one so it shouldn't cause any issues for people without DNS64.