Is there any kind of talk of adding immutability to the core language? "final" keyword doesn't really cut it for objects and immutabililty by convention also is not easy to enforce on large teams.
As to what to do in the meantime - I would suggest that you use either findbugs' immutability detection or Graham Allen's Mutability Detector: https://github.com/MutabilityDetector/MutabilityDetector. Findbugs picks up an @Immutable annotation and checks whether this is the case, whilst Mutability Detector allows you to assert that classes are immutable as part of your unit tests.
I appreciate its not as good as language support, but it is better than relying on an unchecked convention.