If you like Kotlin also look at Ceylon from RedHat. In some ways very similar to Kotlin with null-safety and improved generics.
Random question, but how far along is Ceylon's development compared to Kotlin's? I know they're both relatively new still, but just wondering how feature complete it is so far.
[1] http://confluence.jetbrains.com/display/Kotlin/Null-safety
I hate to say it because I like JetBrains and have given them lots of money over the years but I prefer Ceylon to Kotlin. They are very similar languages but I just like the Ceylon syntax more.