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.
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.
I'm not sure who Tony specifically is in this fanfiction without reading it. It could be an original character created by the author, or perhaps a reimagined version of a known character in a particular fictional universe.
Tony could be the team captain. As the captain, he has a special bond with the team members and might want to do something special to boost their morale. He would know what kind of surprises would really make an impact on them.
In most kerry and tony fanfiction, Kerry is often depicted as brave. For example, in many stories, Kerry is the first to face challenges. Tony, on the other hand, is usually more of a thinker. He comes up with plans to solve the problems they encounter.
In this fanfiction, Tony might also meet Loki. Loki could be either a friend or a foe to Tony. He might try to trick Tony for his own amusement or they could end up working together against a common threat to Asgard.
Tony would likely be defiant. He's not the type to cower in the face of danger. He might immediately start looking for weaknesses in Thanos' setup and try to escape on his own terms.
Well, in the 'Avengers Tony is a God' fanfictions, Tony's character evolution is really interesting. His technological prowess is often magnified to a god - like level. For example, he could have the power to create and manipulate matter at will. This not only changes his relationship with his fellow Avengers but also how he views himself. He may struggle with the newfound power and the responsibility that comes with it, just like in the original stories but on a much grander scale.
One possible aspect could be their adventures together. Maybe they explore new galaxies in the fanfiction. Tony with his technological genius could create amazing devices to help them on their journey, while Starg might have some unique powers or knowledge that complements Tony's skills.