Game development is tricky, as you have to cut corners, content and more to get your game either finished on time or make a better balanced experience. With the very long development history of Final Fantasy XV, it would make sense to have a lot of unfinished content within the base game. Through using an ‘out-of-bounds’ glitch, YouTube user Fotm Hero was able to access the open world area of Niflheim (major location in the world of FFXV). This includes a lot of areas, such as snow-covered landscapes and areas scene in cut-scenes.
Considering this titles long development before it’s 2016 launch, I would hazard to guess that they planed for a second open world but due to wanting a Fall 2016 release, they re-purposed the second open world for story scenes and other such means. With upcoming DLC both paid and free being added to FFXV in 2017, we could possibly see some of this content be completed/used for those pieces of content.
Many famous and iconic games games like Shadow of the Colossus and GTAIII have a wealth of unused assets, areas and models but still end up to be fun experiences. So FFXV having a lot of unused content makes sense in that regard, as making an open world takes a lot of time and not everything made for a game is used in the final release. Regardless, this was a cool discovery and shows that the player-base of FFXV continues to get enjoyment from the game.