This is sort of great, more comments and interest...
My thoughts....
We can make a Basic 2D engine for the first try, since we are at least 4 Java devs...
But moving on probably we will need a better and advanced game engine, since we are starting with 2D there is no big issue, but with 3D I recommend start with Blender GE and then move to JMonkey(Java) or Ogre...
Anyways, I will follow the trend...