Wow, this looks like exactly what I've been looking for. What is the interop story as far as Java/existing Scala code?

A subset of the Java standard library will be reimplemented, so most Scala code that only relies on that subset will just work. Java libraries and Scala code relying on Java dependencies will not work. This is similar to the current state of Scala.js.

Interop with Java seems unlikely unless they keep a JVM tucked away somewhere.

Now that I look at it, they essentially do, it's just that they implement it from a more Scala-centric point of view (e.g. implementing traits directly instead of being layered on top of a Java variant). It seems like it shouldn't be too hard to compile JVM bytecode to run on the same runtime.

As of now, virtually non-existent.