I need this too. I am copying and pasting fields and generating technical debt.
.equals() should be defined as: thisClass.equals(otherClass) + current .equals() logic. That would solve the .equals() problem.
There are several valid scenarios to extend data classes. Composition is not always an option. For example, it is not an option in a web server serving JSON. Why should a API client deal with properties inside objects?
Example: User, Player(User): both classes share name, username, email etc, why should an API class need to access to something like p1.user.name, p1.score? It is a valid user scenario of an API user consuming an object using properties like to p1.name, p1.score, etc. aka using a plain object without sub-objects.