My bet is on Kotlin, and if it doesn't catch on, I'm perfectly fine with Java 8 (I also use Clojure, but you say it's not your cup of tea).
Regarding Java 8, before knowing Scala well, I thought this will kill Scala for good. But I got spoiled by Scala's FP / OO mishmash, I got used to my duck typing, pimp my library, pattern matching, statically typed Frankenscala. perhaps it's not good, but once I figured out when to use which feature, and developed my own "Scala conventions" (not saying it was easy) I feel handicapped writing with other languages. Perhaps Scala needs a book named "Scala, the good parts" and a good Scala Lint as Martin suggested.
Will take a second look at Kotlin still, these guys KNOW programming langues.
EDIT: Kotlin looks great (http://kotlin-demo.jetbrains.com/), until I got to Delegated properties... not much better than Scala's invoke dynamic and lazy vals syntax IMHO. I love the compile to JS feature in any case!
EDIT2: correction, love observable, and delegate by map
http://kotlin-demo.jetbrains.com/?folder=Delegated_propertie...
http://kotlin-demo.jetbrains.com/?folder=Delegated_propertie...
Scala 883,000
Kotlin 21,000Scala 2.12 and Kotlin 1.0 release points should roughly coincide. Scala's got a wee bit of a head start, but if Kotlin delivers on the promise to compile as fast as Java, then for sure they'll gain traction.
Speaking of build times, just got back from the ScalaExchange conference in London (suprisingly large turnout, a little over 400 attendees). Talking with one of the Typesafe team members, it appears that Jason Zaugg (works on the compiler) recently found a way to reduce build times by 20-30%...and that's just for 2.11, good times ahead ;-)
Should point out that incremental builds + sub projects pretty much completely mitigate the "scala is slow" argument, but for deployment Jason's recent enhancements will be a boon to all Scala teams, large and small.
Scala - about 10 years old
Kotlin - not yet released