One popular storyline could be a crossover with Sailor Moon. In this fanfiction, Sakura might team up with the Sailor Scouts to fight a common enemy. Maybe an ancient evil that threatens both their worlds. Another could be a crossover with Harry Potter. Sakura could visit Hogwarts and learn about magic in a different way compared to her Clow Card magic. There could also be a crossover with Pokémon, where Sakura enters the Pokémon world and uses her magic to help in Pokémon battles or to protect the Pokémon from some dark force.
Another concept is that Yugi and his friends discover the existence of the Clow Cards. They team up with Sakura to protect the cards from a new evil force that wants to use them for their own dark purposes. The Yu - Gi - Oh characters could use their unique skills and decks to help Sakura in her battles against the card - related threats.
A fanfiction could focus on Sakura and Syaoran's daily life after the main events of Cardcaptor Sakura. Maybe they are training new cardcaptors. They could be facing challenges like how to teach the newbies about the importance of the cards and the right way to use their magic, all while their relationship continues to develop in this new phase of their lives.
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.
Definitely Naruto and Sakura Kinomoto. They are the central characters in their respective shows and would be the most obvious choices to lead the crossover story. Their different personalities and abilities would create a lot of interesting interactions. Naruto's brash and determined nature compared to Sakura's more gentle yet strong - willed character would make for great story - building.
In Cardcaptor Sakura crossover fanfictions, when incorporating different universes, writers look at the character traits. For instance, if it's a crossover with the world of Fullmetal Alchemist. Edward Elric's determination and Sakura's kindness could be used to create a bond between them. They also think about the power systems. If Sakura crosses over to the world of One Piece, her card magic could be compared to Devil Fruits powers. Writers might also use the concept of alternate dimensions. Like Sakura accidentally falling into the world of Digimon through a dimensional rift and then having to find her way back while helping the Digidestined.
When writing a Cardcaptor Sakura sister fanfiction, it's important to keep the magical elements consistent with the original. For example, if you introduce new cards, they should fit within the existing magic system. Also, explore the emotions between the sisters. Maybe there's some jealousy or admiration that can be developed into a great story. You can start with a simple idea, like the sister accidentally getting involved in one of Sakura's card - capturing missions and then build on that.
A fanfiction could focus on the everyday adventures of Touya and Yukito. For example, they could go on a simple trip together. During this trip, they might encounter some minor magical occurrences that are not directly related to Sakura's card -capturing. It could be a story about their friendship growing stronger through shared experiences and how they support each other in different situations.
One great Cardcaptor Sakura Yue fanfiction is 'The Moon's Guardian'. It delves into Yue's past and his relationship with Sakura in a very heartwarming way. The author beautifully describes Yue's ethereal nature and how Sakura's kindness affects him.