An init system would be useless, there are plenty around (sinit, init, cinit, ...). Same goes for the Window manager. Moreover, we already got venam's 2bwm, which is fully working and just need more people to work on/test features.

My personnal bet would be a game, especially a King of the hill, like core war or this stack exchange game. I was thinking of something where the players could create their warrior in the language they want. Everybody could contribute then (but we'll have to choose a language for the core though).