I figured I would open a topic here so we can discuss this.
I have basic build support in JDK 11 for JME3 working. It is also backwards compatible with JDK 8.
The PR is here: https://github.com/jMonkeyEngine/jmonkeyengine/pull/1059
This does not add modules, it simply makes it so JME3 can be built using JDK 11.
I have tested it on my machine with JDK 8 and JDK 11, on Windows 10.
Can others please test it on Windows, and other platforms with both JDK 11, and JDK 8 to confirm that it works for them.
If you have any feedback, I would love to hear it.