Aside from nullable types, I don't see any other feature that Java could borrow from Kotlin today to improve. Java looks up to Scala and Clojure to get ideas, there are some strong functional programming ecosystems that were bred by these languages, like Typelevel, ZIO, actor-systems like Akka, Datalog queries, contract-based systems like spec2, Stateflow testing, matcher-combinators, generally things that Clojure does differently than the evangelical notion that type-systems are the end all be all. The rest of improvements are on the JVM, like primitive objects (value types) and generic specialization.