Doctrine\ORM\Query should not be directly extendable but it would be nice to decorate query objects and add additional methods. Use cases are e.g. doctrine-fun (see https://github.com/lstrojny/doctrine-fun/blob/master/src/Doctrine/Fun/Query.php) or even cases where users want to add domain specific methods. As Doctrine\ORM\Query is final it is not so easy to decorate correctly. I would propose:

• Add a new interfaces: Doctrine\ORM\QueryInterface that provides a contract for all methods Doctrine\ORM\Query provides
• Add a decorator base class Doctrine\ORM\QueryDecorator as an extension point
• Some for NativeQuery and QueryBuilder