I saw that “free choice of audio input channel” was in the ‘Undecided’ category on the roadmap.
In case you’re looking for a use case, here’s mine:
I have an external microphone hooked up through a mic preamp. To save energy, I don’t have the preamp running unless I’m in a call or recording. Still, I want to have the external mic as the default input in macOS!
By being able to select the audio input channel, I can have Listening Post use my MacBook’s inbuilt mic without having to constantly switch back and forth between the default mic in macOS.