It would be useful to start tracking the JUnit 5 development efforts, trialling the new runner and providing feedback to the JUnit project.

From JUnit Lambda:

The Vision

We’ve identified two main areas to focus on during the upcoming JUnit overhaul:

• Decouple test execution and reporting from test definition and provisioning: This will greatly simplify further evolution of JUnit and allow users to mix test specifications from different test libraries—like JUnit, Spock, ScalaTest and so on—more easily. Other testing libraries should only depend on the test execution/reporting API of JUnit, not on the test definition API (e.g. Runner).
• Rethinking the JUnit’s extensibility story: Runners, Rules, subclassing and other techniques will be revamped into a cohesive set of constructs to enhance JUnit and to allow—if possible—the seamless combination of individual extensions.
• Making use of Java 8 features (Lambdas, Streams, Interface default methods) for better assertions, generating test cases, formulating test hierachies, testing asynchronous code and other stuff: We will provide those extensions in additional libraries to keep the JUnit core compatible with older JDKs.

All development will happen openly on GitHub in order to foster early feedback and detect problems as soon as possible.

See also Prototype Writing Test Cases