Test helper methods.
Example: [Theory] [JsonData("filepath")] public void Test(TestData testData, Expected expected) { // todo }
Also, class to generate combinations of params, somehow. Signature(s) TBD... possibly
[Theory] [Class(typeof(Generator))] -- Standard attrib - generator to work out combinations
or [Theory] [CaseGenerator(typeof(GeneratorConfig))] -- Syntax sugar on above, plus additional optional config (like "exactly these values for this string")