Final Fantasy XIII was a first for the series in that it received not one but two direct sequels. It sounds like Final Fantasy XV may follow the same pattern. According to Square-Enix's Shinki Hashimoto- "because Final Fantasy XV is such a large scale game with a great stories, many stories may come of it at later time."
That could mean DLC, full retail releases, or something else entirely. Just no Final Fantasy XV: All the Bravest please.