Obviously Sakura herself is a very popular character. She is often the central focus of the fanfic, with her growth and adventures being explored.
Some otsutsuki clan members also become popular in these fanfics. For example, Kaguya Otsutsuki might be included in the story, either as a villain or as an influence on Sakura's journey.
Momoshiki Otsutsuki is also quite popular. His powerful abilities and his role as a major antagonist in the Boruto series give a lot of material for fanfic writers. They can expand on his backstory, or create alternate scenarios where he has different interactions with the main characters.
Kaguya otsutsuki is often a very popular character. Since she was the progenitor of the otsutsuki in the Naruto canon, fanfictions like to explore more about her backstory and her relationship with other characters. Another popular one is Momoshiki otsutsuki. His powerful abilities and his encounter with Naruto in the series make him a great character to expand on in fanfiction.
Ryuu is another popular character. He might be portrayed as a strong and noble figure, often having a special bond with Sakura. His skills and character traits are often explored in fanfics, whether it's his combat abilities or his loyalty.
Kaguya Otsutsuki is often a central character. She has a significant role in the original Naruto lore related to the Otsutsuki, and in fanfictions, her backstory might be further elaborated or her powers could be enhanced or explored in different ways.
Kaguya Otsutsuki is often very popular. As the progenitor of chakra, she has a lot of mystery surrounding her, and fanfiction writers like to explore her backstory further, perhaps giving her more complex motives for her actions in the original story.
One good sakura otsutsuki fanfic could be 'Sakura's New Path' where she discovers new powers. It shows her character development in an interesting way.
Often, you'll see characters like a young and curious mage. They might use their magic to travel through time when the sakura petals are swirling around as a sort of catalyst for the time - travel spell. Another popular character could be a historian who is drawn into the past by the allure of sakura - related historical events and wants to experience them first - hand.
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.
In some cases, Karin can be more popular. Her backstory of being associated with Orochimaru and her survival instincts make her an appealing character. Some fans might be drawn to the idea of her as an underdog in the love triangle with Sasuke and Sakura. Also, her sensing abilities and her role in certain plot points in the original series can be explored more deeply in fanfics, which might make her more interesting to some readers than Sakura.
A common theme is power discovery. Sakura often discovers new powers related to the Otsutsuki in these fanfictions. For example, she might unlock some unique abilities that are connected to their ancient lore.