After the story, you might try to unlock all the game's achievements or engage in multiplayer modes if available. Also, you could start a new playthrough with different strategies and choices.
The novel you mentioned might be the book, What to Do If You're Taken by the Missy. The male protagonist, Fang Ye, accidentally became attached to the Missy, Han Niannian, because he was kidnapped. The girl who was later adopted by the assistant should be Han Niannian. I hope you like my recommendation. Muah ~
A possible plot could be that Surtur has a part of his essence trapped in a piece of Asgardian relic on Earth. Tony discovers this relic and when he touches it, he starts having visions of Surtur. Surtur then starts to communicate with Tony through these visions, asking for his help to be fully reborn. Tony is hesitant but realizes that if Surtur is reborn in the wrong way, it could mean the end of the world. So he decides to work with Surtur, but on his own terms. This could lead to a lot of interesting interactions between the two as they have very different personalities and goals.
There are post - main - story quests like exploring new areas that were previously inaccessible. You might also encounter side quests related to the characters you've met during the main story. These quests often provide additional lore, character development, and sometimes new weapons or upgrades.
After the main story of God of War Ragnarok, Atreus is in a position where he has a lot of potential for growth. His experiences during Ragnarok have no doubt changed him. He may be seeking to use his knowledge and skills for the betterment of the realms. For example, he could be using his understanding of the languages and cultures he's encountered to broker peace between warring factions. He also has to deal with the fact that he is now more aware of his godly nature and the responsibilities that come with it. This could lead him to make choices that not only affect himself but also the entire cosmos.
Surtur is a powerful and often single - minded being, focused on destruction and chaos. Tony, on the other hand, is a hero who tries to protect the world. So, Surtur might see Tony's attempts at peacekeeping as a nuisance. Tony would find Surtur's destructive nature abhorrent.
There is indeed free roam available post - story in God of War Ragnarok. This is great because you can fully soak in the atmosphere of the different realms. You can go back to the places where you had epic battles, find hidden collectibles, and interact with the environment in a more relaxed way compared to when you were rushing through the main plot. It adds a lot of replayability to the game.
Engage in combat challenges. The game has various combat arenas where you can test your skills against tougher enemies. You can try different combat strategies and weapon combinations. This not only helps you improve your gameplay but also gives you a chance to earn more XP for leveling up your character.
Replay the game on a higher difficulty level. It will give you a whole new experience as the enemies will be tougher and you'll need to use more advanced combat techniques.
You can explore the realms more thoroughly. There are many hidden areas that you might have missed during the main story. For example, in some of the more remote corners of Midgard, there could be secret chests with valuable loot like new armor pieces or enchantments.