FileBot

The de-facto way for new formats is always to prototype the formats on the GUI first, this works well for scripts too.
I was wondering how complex to add and/or useful to have would be to have a similar environment for the terminal, sort of like a REPL.
Or is it not something feasible/useful?

I think it would be quite difficult. I'll look into existing Groovy tools, and see if there's a smart way to appropriate something for FileBot.