Kotlin is a much better Java, and these days if I need to use the JVM, I will use Kotlin.
However, I do feel that there is a huge missed opportunity in making something that is not just a better Java, but a better language. Kotlin has a lot of annoying limitations, almost all of which are there because the JVM doesn't allow better implementations using first-class JVM functionality.
One of my favourite examples is multimethods which would be an amazing improvement. However, the JVM can't do dynamic dispatch on multiple arguments, so Kotlin will never get it. Sure, if they implement it, calling Kotlin from Java would be more complicated, but that really shouldn't restrict how the language evolves.
Another thing I'd like to see changed is better support for DSL's. Some nice stuff has been done with the limited facilities that are available, but it always seems like I'm fighting the language when making DSL's. As someone who writes a lot of Lisp, I'd really like to see a macro system.