I would like to see a team (or a few teams) try to make the exact same racing game in Java with LWJGL, Java3D, and in C++. Write up the whole development process and benchmark it.
It would provide great insight into where the strengths of each technology lie. Everyone would learn a lot and we would have a racing game in the end...
No commercial guys could afford to spend the time on it.. but maybe if the scope of the game is kept small a dedicated team of java-gaming.org guys could pull it off?
It's what everyone wants to see before they decide which route to go... we are all just waiting for someone else to do it