Final Fantasy 15's first half is a wide-open adventure taking place across the land of Lucis. However, as the party heads into the Niflheim Empire during the second half of the game, the exploration element falls away in favor of a more linear journey.
YouTuber Fotm Hero has used gameplay glitches to get past the boundaries that normally prevent full exploration of places like Cartanica, Tenebrae, and Insomnia. Oddly enough, the video supports fan theories that FFXV was meant to be a much larger game, and that the second half was supposed to be just as open as the first.
The desert area around Cartanica also gives a clue to future DLC additions. Although Cartanica lacked roads when the game released, Patch 1.03 added them for whatever reason. It seems odd to add roads to a section of the map that the character isn't even supposed to visit, leading many to the conclusion that Cartanica will be made accessible and future story DLC will take place there.
If you want to check out some of these normally off-limits areas yourself, you can check out YouTuber Caelum Rabanastre XII/XV's tutorial on glitching out of the game's barriers. Although Square Enix hasn't announced specific release dates for Final Fantasy 15's DLC, which may expand the world map, the Holiday Carnival DLC should be available this month.
If you're still making your way through Final Fantasy 15, take a look at our guides for some helpful info about the game.