I had been following this really awesome looking tile based RTS 3D engine project (think Starcraft2 like) on the jMonkey Engine forums for a while, and it is now fully FOSS including the arts assets.
Don't expect a playable RTS game, but as you can seen in this video of the in-game level editor a lot of work has been done already:
You can contribute to the MIT licensed source on their github page (the author asks for help) and/or head over to the website of engine it uses, jMonkeyEngine, a great project written in Java.
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