In an interview conducted by 4Gamer, the name’s meaning, the game’s length and various other things. The people interviewed were producer Yoshinori Kitase, director Motomu Toriyama, lead designer Yujii Abe, and art director Isamu Kamikokuryou.
The reason the game is not called Final Fantasy XIII-3 is because the team believed this new game to be different enough to justify a new name. Additionally it might have given off the impression that it’s a sequel to Final Fantasy XIII-2.
Generally speaking, this new game opens up different types of gameplay. Final Fantasy XIII is “story driven”, whilst Final Fantasy XIII-2 is “player driven”. This new game on the other hand is “world driven”.
What’s special the “world driven” perspective is that the game doesn’t necessarily revolve around the player. The world is also constantly shifting and changing. It also runs in real-time, coupled with the fact that it changes 24 hours a day.
Regarding the length of the game, it may vary from player to player. You will probably not have the exact same playthrough if you play it multiple of times and therefore you’ll get an “aha” experience when you see events you normally wouldn’t have.