Source-to-source is an interesting example. I think Coffeescript has proven it's pretty possible to get a long way there, though it's undoubtable that it still has some syntactic and semantic design issues.

I think another important example of source-to-source is Lisp macros. Super powerful and when used well and marketed well they can really transform what Lisp means. I also think that when marketed poorly they lead to an explosion of competing "languages".