With the latest video Firaxis has added some explanation how great people work. They are unique to your civ, and must be unlocked by buildings. They also must be built, but will at the end give some pretty significant boni. Join us for the discussion about the video here.
Our forum denizens have also noted that Prussia seems to be in the game, based on this video, but also due to one of the recent new videos by the streamers. This is discussed here.