Just like the previous games from this series, Kingdom Hearts 3 offers additional challenges for those who have completed the main storyline. To answer the question - Yes, you can keep playing the game after you completed the main storyline. What is more, the game unlocks completely new challenges, content, and fights.
What gets unlocked after completing KH3's main storyline?
After completing the main story, you can e.g.:
- Unlock Ultima Weapon. You need to get 58 types of Synthetic Materials, 7 Orichalcum, 2 Wellspring Gems, 2 Pulsing Crystals, 2 Lucid Crystals.
- Complete all Battle Portals. They can be found in every world.
- Defeat a secret boss - Dark Inferno. Open Battle Portal 14 in The Wastelands. You will face an exceptionally powerful boss. We recommend leveling up and getting the Ultima Weapon Keyblade before you face this enemy. Defeat Dark Inferno to receive e.g. Crystal Regalia (+5 Strength, +6 Magic, +16 AP, MP Hastega).
- Photo Challenges. You can keep finding Lucky Marks and taking pictures of them.
- Monster Photo Mission. Take pictures of various enemies for Moogles.
- Gummi Ship missions. You can complete those you haven't done yet. Also, you can take pictures of constellations, hunt for treasures, and customize your Gummi Ship.
- Get all treasures. Each world has hidden treasures and other collectibles, e.g. gold Hercules figures.
- Mini-games. You can replay them and try to get a high score.
That is all. You can also focus on unlocking trophies/achievements, and get the platinum trophy (only on PS4).