Final Fantasy 15‘s open world implementation is sadly limited a bit- but it looks like it may be getting expanded in the coming weeks and months, with future patches. Gearnuke are reporting that recent patches are implementing the second continent in the game, Niflheim, so that it may end up being fully explorable.
As of right now, the continent is only accessible through glitches (you can see the video below showing that), and trying to go to several portions of it results in an infinite loading screen- but there has to be a reason for why this is all being added to the game in the first place. Maybe this will be accessible in the DLC? Or maybe Square Enix will enable full exploration in the post game?
Regardless, it is something intriguing, and should be kept an eye on. Final Fantasy 15 is available now on PS4 and Xbox One.