In an interview with Famitsu, Square Enix game director Hajime Tabata said Final Fantasy XV will be first half open world and second half linear, says a post on Siliconera.
“The entire game structure for Final Fantasy XV consists of both open-world and linear parts,” mentioned Tabata. “The first half keeps going as an open-world, but the story in the second half’s is led by a linear path. That way, you won’t get bored of an open-world as the rest of the game tightens, so we made it in a way that you’ll also get to advance through it as you have in conventional Final Fantasy games. If you play through the first half and only the main route of the second half, I believe the estimated play-time sits at around 40 to 50 hours.”
This approach seems to be a plan to satisfy audiences from Japan and the rest of the world. Many fans complained about FFXIII for being too linear at the start of the game, later changing to a more open world, and having stereotypical characters.
[Source]: Siliconera: Final Fantasy XV Is Only Open World In The First Half While The Second Half Goes Linear.