Don't get me wrong, I'm a long-time Java developer and I've moved into mostly programming in Go the last two years. If I was going to stay on the EE4J platform for the long-term, I think I'd be learning Kotlin (I took the Coursera class on Scala a long time ago but never had a chance to use it). Kotlin feels like writing Python.
Note that I think the long-term prospects for the JVM and .NET are poor since they're such hogs when run in a containerized environment. Perhaps improvements with projects like GraalVM will save them?