Kotlin has plenty of killer features, two of them is a very good OOB IDE support even in Community Edition of IDEA and machine translation of Java code to Kotlin.
The IntelliJ Scala plugin is pretty awesome too and it also offers machine translation of Java code to Scala. It's actually the most widely downloaded plugin for IntelliJ by nearly an order of magnitude. I think Kotlin is a much better option than Java, but the main selling point is simplicity for those who don't want to spend the time learning Scala (entirely valid if if that's a priority for you).