Another motive could be power. Maybe the kidnapper wants to learn his techniques. Since Deidara has such unique and powerful explosive - based jutsu, someone might think that by kidnapping and forcing him to teach, they can gain a great advantage in the ninja world. This could lead to a battle of wills between Deidara and the kidnapper as he refuses to give up his secrets easily.
The motives could be revenge. Somebody who has been wronged by Deidara or the Akatsuki in general might kidnap him to get even. For example, a village that was destroyed by his explosives could seek vengeance.
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.
Revenge is another motive. Maybe someone has a grudge against the Reagan family for past actions. Nicky is an easy target to get back at them. For example, if the family put someone in jail, that person's accomplice might kidnap Nicky to make them suffer.
One possible motive could be ransom. Maybe some criminals knew that Christian was wealthy and thought they could get a large sum of money for his and Ana's release. Another motive could be revenge. Perhaps someone who had a grudge against Christian or Ana from their past decided to take this extreme measure.
One possible motive could be Bellatrix's loyalty to Voldemort. She might think that by kidnapping Hermione, she can get information about Harry Potter and the Order of the Phoenix. Hermione is known to be very intelligent and Bellatrix could believe she has valuable knowledge. For example, Hermione might know the location of Potter or the Order's plans against Voldemort.
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 a case of misinformation. The kidnapper might think Theresa is someone else or has something she doesn't. For example, if there's a prophecy about a girl with certain abilities and they mistake Theresa for that girl. This could lead to a whole series of events where the Titans have to prove she's not who the kidnapper thinks she is while trying to rescue her.
The motive could be ransom. The kidnappers might know that the Tracy family is wealthy and figure they can get a large sum of money for Virgil's safe return.
Well, in a fanfiction where Deidara is kidnapped, it could be that he is taken by a mysterious figure who is actually a long - lost relative or someone with a personal connection to his past. This person may believe that by kidnapping him, they can force Deidara to face his forgotten history or make amends for past wrongs. This could lead to a lot of internal conflict for Deidara as he tries to figure out who this person is and what they really want from him.