Perhaps a fanfic could focus on Sakura's relationships outside of her card - capturing duties. For example, a story about her friendship with Tomoyo growing even stronger. They could go on a trip together and during that trip, they encounter a strange phenomenon that is related to the Clow Cards in a very subtle way. This would test their friendship and also show how the cards are always a part of Sakura's life, even when she's not actively capturing them.
Syaoran Li is quite popular in fanfics. His relationship with Sakura, both as a rival and later as a friend and more, provides a lot of material for writers. There are many fanfics that explore their relationship further, like how they work together to face new challenges in the world of the Clow Cards. Another popular character is Tomoyo Daidouji. Her unwavering support for Sakura and her love for creating costumes for Sakura during her card - capturing missions often feature in fanfics.
One interesting plot idea could be a world - merging scenario. Where the world of Cardcaptor Sakura and a Final Fantasy world start to blend, and Sakura has to use her card - magic to navigate the new and dangerous landscapes filled with Final Fantasy creatures.
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.
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 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.
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.
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.
It depends on how it's written, but generally, it doesn't seem appropriate. Cardcaptor Sakura is a story that is centered around positive values like friendship and courage. A 'diaper fanfic' might involve elements that are not in line with these values or are simply too odd and not suitable for the mainstream audience that the original series targets. If it's just a silly, innocent take on the characters in a different situation, it might be okay for a very small and specific group, but overall it's not a typical or appropriate addition to the Cardcaptor Sakura fan works.
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.
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.