Thanos is extremely powerful with the Infinity Stones. However, Tony Stark has a history of coming up with solutions against seemingly unbeatable odds. In a fanfiction, the author might give Stark an edge by having him discover a hidden weakness of Thanos or develop a new weapon that can neutralize the Stones' power. So, it's possible for Stark to win if the story is written in a way that emphasizes his ingenuity.
One interesting scenario could be Tony Stark using his superior intellect to outwit Thanos in a battle of wits rather than just brute force. He might create a device that can nullify the power of the Infinity Stones for a short time.
In this scenario, it would be a complete shocker. Tony Stark is a hero, and Thanos is a villain. If he were to join Thanos, it might be because Thanos somehow convinced him that his plan was for the greater good. Maybe Thanos showed him a future where overpopulation and resource depletion were so bad that his 'snap' was the only solution. But this would go against everything Stark stands for, so it would be a very complex and dark turn for his character.
Probably the other Avengers like Captain America. He could play a role in trying to save Tony or in the fight against Thanos that ultimately leads to Tony's death. Maybe Doctor Strange as well. Since he has knowledge of the future and the Infinity Stones, he might be involved in some way. He could either try to prevent Tony's death or his actions could inadvertently lead to it.
In fanfiction, their interactions can also be very tense. Tony might try to use his sarcasm and intelligence to taunt Thanos, while Thanos, with his calm and collected demeanor, would simply brush it off. Sometimes, Thanos might even admire Tony's tenacity in the face of such overwhelming power, which could lead to some interesting exchanges between them.
Well, Nebula herself is likely to be one of the main characters interacting with him. They might have some interesting conversations about the technology they both use or their experiences in space.
In some fanfictions, Thanos and Tony Stark have a more personal connection. For example, Thanos has a past encounter with Howard Stark (Tony's father) that changes the whole dynamic. This could lead to a story where Thanos has a different view of Tony, and Tony has to grapple with this new information while trying to save the universe.
One interesting Thanos and Tony Stark fanfiction could be where they have to team up against an even greater threat in the universe. Maybe a new powerful alien race emerges that is set on destroying all life, and Thanos, with his knowledge of different planets and Tony Stark with his technological genius, are forced to put aside their differences. In this kind of story, we might see Tony trying to outwit Thanos at times while also learning from his vast experience in the cosmos.
One group that could be the kidnappers are AIM (Advanced Idea Mechanics). They're always interested in getting their hands on advanced technology, and Stark has plenty of that. The Ten Rings terrorist organization is also a possibility. They've targeted Stark before and kidnapping him again would be a way to make a big statement. And let's not forget some rogue elements within S.H.I.E.L.D. who might think that Stark is a threat and kidnap him to keep him in check.
Perhaps Stark sees the destruction that the constant battles between superheroes and villains are causing to the Earth and the universe. He might think that Thanos' radical solution is the only way to bring a long - lasting peace. He could be convinced that by joining Thanos, he can influence Thanos to be less extreme in his methods. However, this is a very dangerous gamble as Thanos is not easily swayed.
In some fanfictions, the relationship between Tony Stark and Thanos is explored through their interactions with other characters. For example, if Pepper Potts gets involved in some way, it could change how Tony approaches Thanos. Or if Thanos' daughters play a role, it might soften Thanos' stance towards Tony, leading to a more nuanced exploration of their relationship.