Agreed. Even w/o DU's/sum-types/case-classes, there could be a compiler option when matching an instance against a lists of sub-classes that a warning/error is generated if the list of sub-classes does not fully cover the parent class. (It should be fairly easy to write a Rosyln-based plug-in that enforces this.) Typescript allows you to express this kind of thing pretty well -- maybe some of that work will end up migrating to C#. F# used to be the C# feature playground, but that's probably changing now. Not sure if TypeProviders will ever make it...

TP's I don't think have quite borne out the usefullness-to-complexity ratio

Agreed, but anything is better than some 3rd-party tool (that may or may not be integrated into the build) doing one-off ad-hoc code generation. I think any C# meta-programming facility would be a good step. Certainly the TP interface(s) could be simplified for C#.

well there are T4 templates for C# code-gen, although I can't say I've ever been eager to use them.