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.
Regardless, it is something intriguing, and should be kept an eye on. Final Fantasy 15 is available now on PS4 and Xbox One.