pfSense

Currently, in the 1:1 NAT UI, the "External subnet IP" field cannot be populated with options such as "WAN address" or "WAN subnet", but only with a fixed IP address.

This works fine for access setups such as RFC 1483 "routed", but doesn't work for more modern setups such as multiple PPPoE tunnels over the same Ethernet layer to get multiple dynamic IP addresses.

Use cases are pretty much anything benefitting from end-to-end communications such as gaming, home-hosted servers, home automation/surveillance systems, etc.

The workaround I'm currently using comprises creating Port Forward, Outbound and Policy Routing rules, which enormously complicates the setup for artificial reasons (i.e. the scripts for these features support interface addresses while 1:1 only supports fixed addresses).