One area I haven't seen any good solutions for is the interleaving of tests with production code. I think we need better ways to express tests without cluttering the production code and excessive mocking. What I'd like to do, and really can't in any language/tooling that I know of, is to extract some arbitrary subset of the code and surround it with tests or a test harness. I'd also like to specify various injection points for tests right as I'm writing the code in a way that doesn't affect the production code and contributes to its readability. Perhaps this is pieces of server and client code or various services all tested together. When I refactor the code I want the tests to "refactor" with the code. I don't want to need to rewrite my tests... Automatic test generation is another interesting area (beyond the basic table or algorithm stuff we might do in tests today).
It's almost like the "problem" isn't the languages themselves, it's the tooling around the languages. I want to be able to do a lot more than "compile", "run", with my code... I can imagine machine learning driven tooling being able to automate a lot of the mechanical aspects of code writing beyond the simple generate a closing bracket that an IDE can do...