You haven't played the game yet. Even with this map that you are seeing, people are STILL finding completely undiscovered things constantly. The extra content in this game is VERY similar to XII. And mind you, Witcher 3 did the same thing with its first expansion, in that the area for it already existed, but they started working on the DLC AFTER they released the game, which further expanded the area of Novigrad by a healthy amount. I can completely see this being the case for FFXV, in that the entire north side of Lucis is unpopulated, but Galahd is supposed to exist there. Perhaps a DLC they will start to expand the areas they have, create more roads to the north, create more content on the western continent, open up Accordo a bit more, whatever. But you need to relax, the game is HUGE as it is and rivals Witcher in terms of side quest content.
Let me put it this way. People on GAF are finding a bunch of hidden dungeons, that even when you complete the dungeons, there are these huge hidden doors at the end of the dungeon. Oh and those doors are marked on your map as another dungeon. Nobody has a CLUE as to how to open them. Still. To this very moment. Nobody. The story or may not be what everybody wanted, but the open word section is undeniably packed. DLC will only add to that. They didn't cut content from the game, at least not on the gameplay side.
Edit: And listen, this is coming from someone who thought the World Map was going to be so large, that I thought I'd eventually give up exploring all of it. The best thing to do is just keep your expectations in check. They still have created one of the largest JRPG games ever. Even with the size we have. And with online content and DLC, potential for expansion on the game's lore and world is very possible (whether or not you agree to purchase it)