Well, it could be that Lucy has a misunderstanding. Maybe she overhears something that makes her think Natsu is planning to leave the guild without saying goodbye. So, in a fit of panic and anger, she kidnaps him to get an explanation. Then, as the story progresses, they have a heart - to - heart conversation where the misunderstanding is cleared up. It could also be a situation where Lucy is competing with someone else for Natsu's attention, and she thinks that by kidnapping him, she can have him all to herself for a while and figure out how to make him like her more.
Perhaps the rogue is hired by an evil force to kidnap Lucy. She could be the daughter of an important figure or a person with unique abilities that the evil force wants to use against the good side. As the story progresses, Lucy could try to escape or even try to turn the rogue to her side by appealing to his better nature.
One possible plotline is that Natsu starts writing in the diary about his confusion regarding his feelings for Lucy. He might write about how he gets jealous when other guys show interest in her. Another plot could be that Lucy accidentally discovers the diary and confronts Natsu about it, which then leads to them finally admitting their feelings for each other.
Well, in some fanfictions, it could be that Lucy is under a curse. This curse makes her act out of character and she ends up hurting Natsu. Maybe she is cursed to attack the closest person to her, and at that moment, it's Natsu. Or it could be a situation where Lucy is forced to choose between saving herself and Natsu in a dangerous situation, and her choice, even if it's to save herself, could be seen as hurting Natsu. Also, there could be a plot where Lucy is brainwashed by an enemy who wants to break the bond between her and Natsu, and as a result, she causes him harm.
One possible plotline could be that Lucy uses her charm and intelligence to plan elaborate schemes to get Natsu's attention. For example, she might invite him on special adventures that are just for the two of them, where she can show off her skills and bond with him more deeply. Another could be that she gives Natsu special gifts that are meaningful to him, like a scarf she made herself which has some magical protection. Or perhaps she starts dressing in a way that she knows Natsu will find appealing, not in a vulgar way but in a way that highlights her beauty and confidence.
Perhaps Natsu kidnaps Lucy as a prank that goes wrong. He initially thinks it'll be a funny joke, like taking her away during a guild party just for a bit of chaos. But then things get out of hand. Lucy might be really angry at first, but as they are alone together, they start to have some deep conversations and understand each other better.
It could be a new, powerful dark wizard who has a grudge against Fairy Tail. This wizard might manipulate Natsu into kidnapping Lucy to create chaos within the guild. The dark wizard could be after some powerful magic that only Lucy can access, and by kidnapping her, they hope to force her to give it up.
One interesting plotline could be Lucy and Natsu going on a quest to find a lost dragon egg. Along the way, they face many challenges and their bond grows stronger. Another could be Natsu suddenly losing his dragon powers and Lucy having to find a way to get them back, which involves delving into some ancient magic lore.
Another plotline could be that they are attending a guild - sponsored camping trip. Their tent gets damaged in a small storm, so they have to share a different, smaller tent. They start off a bit awkward but then end up sharing stories of their childhoods and how they came to be in the guild. As the night goes on, they gradually fall asleep, with a new - found closeness in their friendship.
One possible plotline could be that Natsu and Lucy go on an adventure to find a special lemon tree that is said to have magical properties. Along the way, they face challenges that make them realize their feelings for each other more deeply. For example, they might have to protect each other from dangerous monsters. As they search for the lemon tree, they share tender moments and by the time they find it, they confess their love under the shade of the tree.
It could be another guild that sees Natsu as a threat. They might find out about Lucy kidnapping Natsu and try to use the situation to their advantage. For example, they could try to steal Natsu's magic or get information out of him while he's in a vulnerable state. Another possible antagonist could be a character who has a grudge against Lucy. They might try to stop her from getting away with the kidnapping or use it as a way to get revenge on her.