Hi there,
I don't have any plans to extend the IDL, but feel free to fork the project and start experimenting.
My primary concern with extensions is that we implement those extensions consistently across the language bindings. Things like numeric ranges and string lengths would be easy to add. Adding regular expressions is trickier since the regex implementations have some differences across languages.
A good next step might be to propose some concrete IDL changes. A github issue on the main barrister repo is probably the best way to do that since.
cheers
-- James