Roslyn's parser and syntax tree is pretty amazing. You can recreate the precise source text from the parse tree up to whitespace. This sort of "bijective parsing" is truly incredible and probably one of the cooler innovations I've seen in parsing technology. I can see a bunch of really interesting ideas that you could do with bijective parsing. For instance, imagine Rails style boilerplate generation but done at a semantics aware, within file basis. You could conceivably have a code generator that finds a class, introspects if it has the corresponding method, then generates it if not.

Or imagine syntax reformatting but purely locally. Or semantically aware git diffs that can actually compare the underlying parse trees instead of just raw text. There's so much cool stuff you can do. I wish every language had a parser like this.