You need not wait for invokedynamic to try Clojure. There is hardly any dynamic dispatch that can take advantage of that AFAICT.
Lambdas/Closures when they come to Java will certainly be backward compatible with the JVM (and hence just syntactic sugar) and will implement Runnable/Callable in some form.
Few points I can recollect (without answering "which language is most applicable...."):
1. Future of Java-the-language is limited by backward compatibility. This imposes a barrier on the scope of innovation in the language.
2. #1 can be helped by the newer JVM languages.
3. Current trend to increasingly adopt Ruby, Python or PHP for turnaround time is the result of a momentum and has to mean something. There is clearly demand for faster turnaround time in some segments.
4. The JVM languages that can reach feature parity with Ruby/Python/PHP ecosystem and tooling will have a better chance of adoption IMO.
5. The next Java is certainly not Java alone. Probably a mix of languages, depending upon which ones have a better VM, tooling, libraries, turnaround time etc.