Just wanted to share what I know, take this as you want.
There are multiple features that will be added for free the day the season pass episodes becomes available, and those are huge.
There is a secret DLC but it may take a while, I don't know much about this one, I just know that it may add Insomnia before the events of the game, but this would be a different DLC so they are thinking about releasing second Season Pass (2018) or just releasing this DLC alone and priced apart from the main Season Pass. If they decide to go for the second Season Pass then that's gonna give us another year of working in XV (almost till August/September 2018). If they decide to only release the Insomnia DLC alone then that will take us until April 2018.
Without going further this is what I know.
Schedule:
Update 1.06: Chapter 13 adds Gladio as a playable character to this chapter, you go with Ignis the whole time and he needs to be protected. Some new cutscenes are added.
DLC Free Booster Pack: It's coming the first or second week in march, adds the magitek suit for the 4 with a new design.
DLC Episode Gladio: Adds a full 2-5h Gladiolus experience (time depends on the player) with a new dungeon along with Cor Leonis to make Gladio stronger, by the end you encounter Gilgamesh, a big gear with tons of blades. There will be some backstory about Clarus, Regis, Cor, Iris and Gladio. Also this dungeon will be added to the main game, you can go there with you party but the access to the final boss won't be available, it's only for episode gladiolus. This dungeon won't be accesible unless you have the Gladiolus Episode. More dialog than cutscenes and a lot of challenging enemy battles.
Update 1.07: Some point in May, this one will add a bestiary feature, in order to complete this journal you need to take pics with promptos' ability Snapshot and the enemy needs to use every single move (even special ones) in order to complete each profile, if the enemy didn't use a move against you it won't appear in the list, the overall bestiary has a % of completion and the team is working on expansion trophies/achievements (for bestiary, fishing, recipes and completion of the side quests)
Update 1.08: This one comes in June, will add the ability of going out of the tracks with the regalia using an update with a new mission from Cindy. This mission feature Prompto alone. Also adds Prompto playable to chapter 13 by the moment Noctis is with the crystal. This sequence will be alternated between Noctis gameplay and Prompto gameplay.
