Use case: I'm on an IPv6-only network that optionally provides outgoing IPv4 support through use of NAT64. There is no automatic route for that, but internal resolvers use DNS64 to map address accordingly.

Problem: But now I want to use NextDNS, and of course even if I only connect to IPv6 endpoints of NextDNS, I will still get answers with IPv4 addresses which my device cannot connect to successfully because there's no route.

Idea: In my profile settings, if there was an option to enable DNS64, then the problem would be solved. By default the prefix 64:ff9b::/96 should be used, but it would be even better if also a custom internal prefix could be configured. We use our own custom subnet for DNS64. 

Additional resources: The Google Public DNS also provides such a service, see https://developers.google.com/speed/public-dns/docs/dns64 -- however they only support the default prefix and not something custom.