Here's a plot idea. The FEM OC is a descendant of a powerful magician who was involved in the creation of the Clow Cards. She comes to Tomoeda to find out more about her heritage. Along the way, she meets Sakura and they form a bond. However, there is a dark force that wants to use the OC's connection to the Clow Cards for its own evil purposes. So, Sakura and the OC have to fight this force while also exploring the OC's past and her powers. This plot can involve a lot of mystery, adventure, and character development for both the OC and Sakura.
One idea could be creating an OC who is a new transfer student at Tomoeda Elementary. This OC could have a special power related to a different set of cards from a far - off land. They initially struggle to fit in but end up befriending Sakura and helping her with card - related adventures in a unique way.
Perhaps in the fanfiction, there's a parallel universe where Sakura and Syaoran are enemies. They have to figure out what has gone wrong and how to set things right. This would involve a lot of self - discovery and exploring the nature of their relationship in different circumstances. They would need to find a way to break the spell or whatever is causing this rift between them and restore their normal relationship in their own universe.
One idea could be that Sakura discovers a new set of Clow Cards with powers she's never seen before. For example, cards that can manipulate time in different ways like freezing time in a small area or making time move faster for a specific object.
Well, a plot could be where Touya and Yukito swap personalities for a day. Yukito becomes more assertive like Touya usually is, and Touya becomes more gentle and reserved like Yukito. This would lead to some really funny and awkward situations as they interact with their friends and family. Their friends would be really confused at first but then start to see different sides of them.
Maybe there's a fanfiction where Li Lemon gets transported to an alternate dimension where he's the only one who can remember Cardcaptor Sakura. He has to find a way to get back to their own world while also dealing with the strange new rules and challenges in this dimension. And Sakura, on her end, is desperately trying to find a way to bring him back.
One idea could be a crossover where Naruto and Sakura from Cardcaptor Sakura meet in a magical world that combines elements of both their universes. They could team up to fight a new kind of threat that is causing chaos in both the ninja world and the world of the Clow Cards.
To create a great FEM OC for Cardcaptor Sakura fanfiction, look at the existing themes in the series. Magic is a big part, so her magic should have its own flavor. Maybe she has a magic item, like a bracelet that gives her the power to manipulate water. Her relationships are also important. She could befriend Tomoyo easily as they both have an interest in the magical world. And her character arc could involve her learning to control her powers better and becoming more confident, much like Sakura's own journey.
A cool concept would be a plot where the villains from both universes team up. Orochimaru from Naruto and Eriol from Cardcaptor Sakura could form an alliance. Then Naruto and Sakura have to work together, along with their friends, to defeat this combined threat. They would have to combine their different powers and strategies, like Naruto's shadow clones and Sakura's use of the Sakura Cards.
One idea could be Sakura discovers a new type of clear card that has the power to time - travel. She could use it to go back to the early days of her card - capturing adventures and meet her past self. Another idea is that a new character arrives in Tomoeda who has the ability to create dark versions of the clear cards, causing chaos for Sakura.
One plot idea could be that the fem OC discovers a hidden power within Mew that no one else has noticed. She then has to protect Mew from an evil organization that wants to use this power for their own gain. Another idea is that the fem OC and Mew form a special bond that allows them to communicate telepathically, and they use this bond to solve mysteries in the Pokemon world, like finding lost Pokemon or uncovering ancient secrets.