The game industry tends to treat games as a form of media that you consume and then buy the next installment. What happens to our culture and business when people instead start playing a single game for years or decades?
Free-to-play game design is ongoing and iterative by nature: the launch version usually only contains a core set of features. Based on your predictions and the feedback you receive from players you want to add or extend features later on. This leads to many small game design documents.