One possible reason could be to test Tony's loyalty. Thanos might want to see if Tony is truly committed to his own self - interest over protecting Peter. Another reason could be to break Tony psychologically. Tony has a father - like relationship with Peter, and forcing him to harm Peter would be a huge blow to Tony's mental state.
Perhaps Thanos sees Peter as a potential threat in the future, and by making Tony push Peter, he wants to eliminate that threat while also causing internal conflict within the Avengers. It could also be part of Thanos' grand plan to sow chaos and discord among the heroes. He knows that this act would not only affect Tony and Peter but also the entire team dynamic.
If he's unable to avoid it, Tony would be extremely conflicted. His face would show a mix of horror, guilt, and anger. He might yell at Thanos, blaming him for making him do such a cruel thing. After pushing Peter, Tony would likely be in a state of shock and self - loathing, feeling like he has failed Peter and himself.
Well, one possibility could be that Tony sacrifices himself in a final battle against Thanos. Maybe he uses some new, untested technology that ultimately drains his life force but also manages to defeat Thanos. Another idea could be that Thanos, in a moment of unexpected mercy, spares Tony's life initially but then something goes wrong in the aftermath, leading to Tony's death. For example, perhaps the Infinity Stones' power backlash affects Tony in a way that kills him.
In a 'Tony kills Thanos' fanfiction, the plot could start with Tony being haunted by his previous failures against Thanos. He decides to go on a solo mission to study Thanos more closely. He discovers a way to disrupt Thanos' connection to the Infinity Stones. With this knowledge, he devises a plan. He lures Thanos into a trap where he has set up a device that can nullify the power of the Stones. Then, in a tense battle, Tony uses his own advanced technology and combat skills to fight Thanos without the Stones' advantage and finally manages to kill him. This could be a story of redemption for Tony, showing his determination and ingenuity in the face of seemingly insurmountable odds.
One possible plotline could be Thanos trying to win Tony over while still being true to his villainous ways. Another could be Tony using Thanos' feelings to his advantage in a battle.
Maybe Peter has some urgent news to share. For example, he might have discovered a new threat to the city or a problem related to one of their projects. He could also be in trouble himself and needs Tony's immediate help.
Peter could use his agility. Since he's super - agile, he can dodge Thanos' attacks easily. Then, when Thanos is off - guard, Peter can strike. For instance, he can web up Thanos' hands so he can't use the Infinity Gauntlet properly.
One consequence could be that Peter Quill Parker becomes a hero. People all over the universe might look up to him. But he may also face some jealousy or envy from other powerful beings.
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.
It could also be due to external pressure. The media or some powerful figures in the fanfiction world might force Tony to stay away from Peter. They could spread false rumors about their relationship or use their influence to separate them for their own gain.
Another aspect could be the emotional turmoil for the other Avengers. They would surely be scrambling to find Tony. Captain America might feel especially guilty, considering their past differences. The team would have to put aside their differences completely to rescue Tony. They might start by gathering intel on Thanos' whereabouts and formulating a plan that involves their unique skills. For example, Black Widow could use her espionage skills to infiltrate Thanos' base, while Thor could provide the brute force needed in a direct confrontation.