Well I am prototyping an FPS for quite some time now... and it's nothing like modern shooters. It has/ should get old school low poly graphics, no autoheal, no military weapons, no cover system, non-linear levels and a free movement script, much like the Superku movement but in 3D.
I doubt that the game will ever get finished, but working on it from time to time is quite fun - and that's the most important part about it.