Not really in any order:
Add multibyte handling to fields, if they get a multibyte string, try encoding it into UTF-8 B first, if this fails, throw an error.
Cleanup the treetop parsers.….…. do I really need that many entrance files?
Simplify the relationship of Headers and Fields. Doing too much of the Field work in the Header class on instantiating fields. Header should just say “Field, do it!”