I'm thinking a game for the Library like Containment Breach would be a good idea, but it would naturally be quite different. If we ever make one, I think it should have a focus on exploration and wonder, this being the Library, but I would also suggest puzzles and an element of horror in order to keep things interesting. Or more interesting, at any rate. I'd like to see some other ideas for a first person game here, including how to actually make one.
I'm thinking we could use either the Unity engine CB used or the Source engine, as seen in countless custom levels for the Half-Life and Portal games (not to mention Garry's Mod). The latter, AFAIK, would have the advantage of being playable in the aforementioned games and being able to use their assets, not to mention a proven physics system. Unity might have comparable physics but I'm not sure.
The end goal of the game could be reaching the Library. And here's an oddball idea I had: the player coming across a farmhouse, hearing an emergency broadcast and having to seek shelter because a tornado is bearing down on the house.