A modern programming language that makes developers happier.

kotlinlang

As far as semantics for Java and JS, it might be an ugly opinion, but mine (in general) is that it should be optimized for Kotlin. Java and JS should be used to crappy APIs. Why bog down Kotlin development to support them?

I know interop a big part of Kotlin, but if Kotlin becomes good enough of an abstraction for multiplatform (and it's already so close), then it should be the primary focus. Consumers using other languages should be a secondary focus. Not to say they should be ignored, but build better support later, if it doesn't impact the Kotlin experience.