If methods and parameters were annotated for nullability (ie: either @Nullable or @NotNull) it would help Java programmers who use static analysis tools, and would be a huge help to Kotlin programmers using the Java SDK.

Nullability annotations in Java unfortunately suffer from a bit of a too many standards problem, but JetBrains has a list of nullability annotations supported by the Kotlin compiler as well as other advice that should help with migrating to using nullability annotations.