In a fanfiction, the motives can be as diverse as the creativity of the author. It could be that Manny Ruiz believes Sam Gh has some sort of secret or power that he wants for himself. Perhaps it's a power struggle within a particular group or organization they are both part of. Manny might think that by kidnapping Sam Gh, he can gain an upper hand. Or it could be a matter of love gone wrong. If there is a love triangle or some sort of unrequited love situation between the two characters or involving other characters related to them, that could lead Manny to kidnap Sam in a fit of passion or desperation.
It could be for ransom. If Sam Gh is a wealthy or important character in the story's universe, Manny Ruiz might kidnap him to get money or some valuable item in exchange for his release. Another motive could be that Manny Ruiz is being controlled or manipulated by someone else who has an interest in Sam Gh. This third party could be using Manny as a pawn to achieve their own goals, like getting information from Sam or eliminating him as a threat. There are also motives like jealousy, if Manny Ruiz is jealous of Sam Gh's status, relationships, or abilities, he might resort to kidnapping as a way to assert power over him.
One possible motive could be revenge. Maybe Sam Gh did something to Manny Ruiz in the past that made Manny want to get back at him.
I'm not sure specifically if there is a fanfiction with that exact plot. You could try searching on major fanfiction websites like Archive of Our Own or FanFiction.net.
It could be that Naruto has lost his sanity temporarily. The stress of being a hero, the constant battles, and the losses he has endured might have taken a toll on his mental state. In this state of instability, he may see Sakura as a symbol of something he wants to preserve or protect, but his way of doing so is through kidnapping. Or there could be a new villain in the story who has the ability to make Naruto act out of character, and this villain's plan involves Naruto kidnapping Sakura.
Perhaps Fleur is being controlled or influenced by a dark force. In the Harry Potter universe, there are many dark magic entities that could manipulate someone into kidnapping Harry. It could be that she is under the Imperius Curse or some other form of mind control. Another reason could be that she has some sort of personal vendetta against Harry or those close to him, and kidnapping him is her way of getting revenge. She might think that Harry has wronged her in some way, whether it's real or just in her deluded perception.
Well, it could be out of jealousy. Maybe Dean feels like he's losing Ginny to someone else and in a moment of madness, decides to kidnap her. But this is all just speculation within the wild world of fanfiction.
It could also be for power. Perhaps Stiles has some knowledge or ability that Theo wants to use for his own gain. In the show, Stiles was quite smart and resourceful, so Theo might kidnap him to get access to that intelligence or those skills. In some fanfictions, Theo might be trying to use Stiles as a bargaining chip to get something from the other characters like Scott or Derek.
Well, in this kind of fanfiction, Doc Oc could kidnap Spiderman to force him to reveal his identity. Doc Oc might think that knowing who Spiderman really is would give him an upper hand in their ongoing battles. It could also be that Doc Oc wants to study Spiderman's powers. He's a scientist after all, and Spiderman's abilities are quite unique. He might believe that by kidnapping and examining Spiderman, he can find a way to replicate or counter those powers.
One possible motive could be to force Naruto to understand his own power. Sasuke might think that by kidnapping Naruto, he can push Naruto to a situation where he has to tap into his full potential. Maybe Sasuke feels that Naruto has been relying too much on his friends and needs this shock to grow stronger on his own.
It could also be a matter of desperation. Perhaps there is a threat to the village or someone she loves dearly, and she believes that Naruto holds the key to solving the problem. But instead of asking him directly, she kidnaps him in a hasty and ill - conceived plan. This could lead to a lot of drama and character development as Naruto tries to understand her actions and they both work towards resolving the real issue.
One possible motive could be that in the fanfiction, Adrien is being controlled by some dark magic or an evil force. This external influence makes him act against his true nature and kidnap Marinette. For example, perhaps a villain has cast a spell on him to use him as a tool to get to Marinette, who might have some special power or knowledge that the villain desires.
One possible motive could be jealousy. Maybe the kidnapper woman is jealous of the saleswoman's success or the attention she gets from customers. Another motive might be related to a personal grudge. For example, the saleswoman might have unknowingly offended the kidnapper in the past. It could also be a case of misinformation. Perhaps the kidnapper has been wrongly informed that the saleswoman has something she wants, like a valuable item or some important information.