One interesting plot could be that Gaara's little sister has a special power that she has to keep hidden from the village at first. This creates a lot of mystery and tension as she tries to navigate her life without being discovered.
Perhaps she goes on a journey to find a way to heal Gaara from his inner demons. Along the way, she meets various characters who either help or hinder her progress, and she has to use her wits and kindness to overcome obstacles.
One interesting plot could be Hisoka's sister getting involved in the Hunter Exam. She might use her unique skills, perhaps some that are similar to Hisoka's but with her own twist, to navigate through the challenges. For example, she could have a special ability to read opponents' intentions like Hisoka, but she uses it in a more empathetic way to avoid fights when possible.
One interesting plot could be her going on adventures with Natsu and his friends. She might have unique magical abilities that help the group in their quests.
Temari and Kankuro, Gaara's siblings, are likely to be in the fanfiction as well. They could play protective roles towards their little sister, or they might have some conflicts with her as she tries to assert her own identity. And there could be some antagonists, like rogue ninjas from other villages who see Gaara's sister as a target for some reason.
She could also be a key figure in the political intrigue of Camelot. As Arthur's sister, she has a certain status. There might be those who try to use her to get to Arthur or undermine his rule. So she has to navigate the dangerous world of court politics, using her wits and charm to outwit her enemies and protect Arthur and Camelot.
One interesting plot could be that female Naruto, with her determination and positive energy, teams up with Gaara. They go on a journey to help a small village that's been affected by a strange disease. Along the way, they learn more about each other's pasts and how they overcame their own hardships. Gaara, with his calm and collected nature, and female Naruto, with her exuberance, make an unlikely but effective duo.
Perhaps the sister has a special ability that Gaara is intrigued by. This ability could be something related to healing or a unique form of chakra control. Gaara, being interested in power and different abilities, starts to spend more time with her to learn about it. And during this time, they grow closer. For instance, she might be able to heal the emotional scars that Gaara has carried since his childhood, and this becomes a turning point in their relationship.
Maybe in the fanfiction, their brotherhood is tested when they are forced to make difficult choices. For example, a powerful villain offers one of them a deal that could save their village but at the cost of betraying the others. Gaara, with his experience of being an outcast and then a leader, could be the one to find a solution that doesn't involve betrayal. Naruto would be all for sticking together no matter what, while Sasuke might be more tempted by the power the deal offers.
She could make him more empathetic. Gaara has been through a lot, but his sister's presence might make him more aware of others' feelings. He'd see her innocence and want to protect it, which would make him more attuned to the needs of the people in the village.
One possible plot could be that the sister gets into some serious trouble in the Vampire Diaries world, like unknowingly making a deal with a vampire that endangers the family. Her older sibling decides to spank her as a form of punishment to teach her about the dangers. Another could be that she breaks a family heirloom that has some sort of protection spell in the vampire - filled town, and the spanking is more about her learning respect for the family's heritage in this supernatural context.
One plot could be Naruto protecting his little sister from the threats that still lurk in the ninja world. Since he has grown older and more experienced, he uses his skills to keep her safe.