2016 might've been the year that Final Fantasy XV launched on Xbox One and PlayStation 4, but maybe 2018 is the Year of Final Fantasy XV. Square Enix has more DLC episodes planned, the Windows Edition, and the Pocket Edition planned for release this year. Now, the Entertainment Software Rating Board (ESRB) has rated Final Fantasy 15 Royal Edition for PlayStation 4 and Xbox One.
The ratings summary doesn't add any new information about the game, but it's likely that Royal Edition is the (current) complete package for Final Fantasy XV. Episode Ignis was the last planned downloadable content (DLC) when the game launched, so it's probably time to make a retail release that contains everything to date. My guess is you'd be getting Episodes Gladiolus, Prompto, and Ignis alongside the Comrades multiplayer mode.
Otherwise, I have no clue what this new edition of Final Fantasy XV might contain. We'll have to wait until Square Enix actually announces it.