One possible motive could be money. The Stark family is wealthy, so the kidnappers might be after a large ransom. They could think that by holding Peter captive, they can force Tony Stark to pay up a huge sum.
Another motive could be revenge. Maybe some business rival of Tony Stark's wants to get back at him. They figure that by kidnapping his son, they can cause him great emotional pain and also try to disrupt his business operations. For example, if Tony is distracted by the search for Peter, he might make mistakes in his business deals.
One possible motive could be to gain power. Since Goten is a Saiyan, his power could be harnessed in some way. For example, they might want to extract his ki or use his DNA for some evil experiment.
One possible motive could be to use Hermione as a bargaining chip against Harry Potter. Since Hermione is his close friend, the kidnappers might think they can force Harry to do something, like give up a powerful magical item or valuable information.
One possible motive could be to gain power. Natsu is a Dragon Slayer, and his magic is very powerful. The kidnappers might want to extract his magic or use him as a weapon. Another motive could be for revenge. Maybe Natsu has unknowingly wronged someone in the past, and they are out for vengeance. Or perhaps they are working for a greater evil, and Natsu is seen as a threat to their plans, so they want to get rid of him by kidnapping him.
One possible motive could be power. The kidnappers might think that by having Princess Peach, they can control the Mushroom Kingdom. Since she is the ruler, her capture could give the kidnappers leverage to make demands on the kingdom's resources or governance.
It could be some disgruntled former employees of Stark Industries. They might feel that they were wronged by Tony in some way, like being fired or not getting the credit they deserved for their work. And they see kidnapping him as a way to get revenge. Peter might just be collateral damage since he is associated with Tony.
The kidnappers could be motivated by jealousy. They might see Kuroko's skills and his importance to Seirin and want to take him out of the picture so their own team can have an advantage. For example, a rival team that has been losing to Seirin might think that without Kuroko, Seirin will be easier to defeat.
One possible motive could be ransom. If Himawari is from a well - off family, the kidnappers might be after a large sum of money. Another motive could be revenge against someone close to Himawari, like her parents. Maybe they have some sort of vendetta and think kidnapping her will be a way to get back at them.
Another motive could be related to Kirishima's unique quirk. Maybe the kidnappers are interested in studying or stealing his hardening quirk for their own malicious purposes. They could be part of an illegal quirk - trading organization or some sort of mad scientist's group.
Another possibility is some disgruntled former employees. Maybe they feel they were wronged by Stark and want to get revenge. They know his routines and weaknesses well enough to plan a successful kidnapping.
In a dark alley, young Peter Stark was snatched. The kidnappers, a group of villains, had no idea who they really had. Peter, though just a child, was already brilliant. He used his wits to leave small clues along the way. His father, Tony Stark, was in a panic. He launched all his resources to find his son. Meanwhile, Peter was trying to outsmart his captors, waiting for the moment to escape and be reunited with his family.