This is not a high school project and the work of one person limited to thier new access to a new project assigned in thier new class.
We are talking about a group of professionals who have been doing this for years, have played games before doing this for years, and have access to a ton load of quality ideas.
See the difference?
The only thing a dev team has to do is basically make sure the idea on paper can work with the engine, and is balanced with testing for the classes. This does take time, but it should not limit quantity by a huge margin.