It makes sense that some field types would have a different validation API. You could allow some basic length/range tests on strings and numbers, regex, custom formulas, schema validation for JSON/XML, etc. Personally, I’d be happy with just regex to start, but an actual UX would be good for data modelers with less programming experience.
It would also be good UX to have some presets for common things, like URL-style slugs. Pretty much every public-facing model I have uses slugs (valid, readable URLs are important), and my users break them all the time. Never trust human input :-(
Also, to be clear, validation really needs to happen at the API level as well as the editor UX. If I call a mutation, I’d like to to get back usable, readable error messages from the GQL endpoint. Ideally I could customize that error message, with extra hints to help people figure them out.