Another reason could be that Tsunade is trying to use Naruto for some sort of political leverage within the village or against other villages. She may think that having Naruto in her control gives her an advantage, but as the story unfolds, she realizes the error of her ways.
Sasuke might kidnap Naruto because he wants to prove his superiority. By taking Naruto captive, he can show that he is stronger and more in control. It could be a power play, where Sasuke is trying to assert his dominance over Naruto once and for all.
Another reason could be to protect her from a prophecy. In the world of fanfiction, there could be a prophecy that foretells Renesmee's doom if she stays with the Cullens. So Jacob, thinking he's doing the right thing, kidnaps her to keep her away from that fate.
In some fanfictions, the Order might kidnap Hermione because they believe she has some sort of dangerous knowledge that could fall into the wrong hands. They think that by taking her into their custody, they can protect that knowledge and keep her safe from the real enemies who might want to use her.
In some fanfictions, Vegeta could be under the influence of some external force. Maybe an evil entity has convinced Vegeta that kidnapping Goku is the key to achieving ultimate power. Vegeta, being power - hungry as he is, falls for it and kidnaps Goku. But later, he realizes his mistake and has to make amends, which can lead to an interesting story arc of redemption and friendship building between the two Saiyans.
Maybe she wants to disrupt the balance in the ninja world. Naruto is a key figure in maintaining peace and unity. By kidnapping him, she can create chaos and make it easier for her to regain control over the world she once ruled.
Vlad could kidnap Danny in fanfiction for revenge. Maybe he blames Danny for something, like ruining his plans in the past. Or perhaps he wants to use Danny as a bargaining chip against the other ghosts. He might think that by having Danny under his control, he can gain more influence in the ghost world.
Well, it could develop through shared experiences. For instance, if they both go on a diplomatic mission to another village. During the journey, they face difficulties like hostile ninja attacks or harsh weather. They have to rely on each other's skills. Naruto's combat skills and Tsunade's strategic thinking complement each other. This cooperation makes them closer and over time, love might bloom between them.
It could be due to the current situation in the village. There might be threats that required Naruto to stay. Tsunade, as the Hokage, had to consider the overall safety and stability of the village. So, she denied Naruto's request for the training trip to ensure he was available in case of any emergency.
It's not a very common theme. Naruto is usually depicted as a hero who respects and protects his comrades, and Tsunade is an important figure in the Naruto universe. However, some fanfiction might explore such a dark theme to create an alternate universe or a shock factor. Maybe the author wants to explore how Naruto could be corrupted or misdirected in a different storyline.
He could kidnap her because he believes she's in immediate danger. Natsu is very impulsive and might not have time to explain things properly, so he just takes her away to keep her safe.