My feature request is to have private DNS zones. These zones would be internal only within the fly.io organisation and not exposed publicly. They would take precedence over public dns records and you would be able to create a private zone for any domain.
e.g. Org1 could create a public zone for example.com with an A record for www.example.com pointing to 1.2.3.4, and create a private zone for example.com with an A record for www.example.com pointing to 172.168.2.5. When querying for www.example.com from an app inside Org1 the response would be 172.168.2.5, but when querying from inside Org2 or from the public internet the response would be 1.2.3.4
This would allow for having something like api.myapp.com resolving to the anycast IP for public queries but resolving to the private 6pn load balancer address when queried internally allowing to trafffic to essentially stay within the organisation.