I'm not sure I have time to join this, but I am considering working on an FPS project myself. I have a few ideas of different game choices, but the FPS one seems the most promising right now. I have about 2 months to decide, before I graduate school. I'd be willing to share some of my code, that is not an issue, but the project will not be open source (might release the source like a year or two after release similar to DOOM, once I have made some money).