I have stumbled upon your repository a week ago and I have to say, great work and great ideas!

Another thing I thought about is integrating formatting for fields using a similar system. ISO-8601 dates comes immediately to mind but also number and currency formatting are other examples.

Probabilistic enums is another thing that I can think of that might be useful for fallback values, I am pretty sure there's a lot of work that can be done in this area, also for other parser kinds

related and highly recommended resource is https://github.com/mkuchnik/relm and https://arxiv.org/abs/2211.15458. It is a similar system used to validate LLMs using regexes, however built for completely different use cases. I imagine integrating regex checks to the output fields can also have a lot of use cases.