The Shugo Chara anime didn't complete the manga. There were still parts of the manga story left unexplored in the anime adaptation. This is quite common in many anime-manga pairs.
I'm not sure of a specific one off - hand. But you can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Shugo Chara Amu pregnant' in their search bars and you should be able to find some relevant stories.
Sasuke could be a great main character. He already has a complex backstory and a lot of internal conflict. In a Naruto Shugo Chara fanfic, his Shugo Chara could represent his search for revenge or his later - in the series - desire to protect the village. His journey of self - discovery could be enhanced by the addition of the Shugo Chara concept.
One idea could be a crossover where the characters from Naruto and Shugo Chara meet in a strange new world. They could team up to fight a common enemy that threatens both their universes. Another idea is to focus on a character from Naruto who suddenly gains a Shugo Chara. For example, Naruto could have a Shugo Chara that represents his dream of becoming Hokage in a more physical and guiding way.
Love or budding romance is also likely to be a theme. Fans might write about the potential romantic feelings between Rima and Nagihiko. This could involve things like blushing moments, secret glances, and the slow development of their relationship.
One idea could be a male OC who has a unique Shugo Chara that represents his love for art. He could use his art skills and the power of his Shugo Chara to help his friends when they face difficult situations at school, like bullies or tough exams. Another idea is a male OC from a different country who comes to the Shugo Chara world and has a Shugo Chara based on his homeland's culture. This could lead to interesting interactions as he shares his different perspective with the other characters.
A possible plot is a time - travel plot within the Sakura Haruno Shugo Chara fanfiction. Sakura could be sent back in time to when the first Shugo Chara was created. There, she could learn the true origin of the Shugo Chara and use that knowledge to solve a current crisis in her world. Maybe she discovers a long - forgotten power that can save her friends and the Shugo Chara world from destruction.
One possible fanfiction could be about Amu's journey while pregnant and how she still tries to maintain her normal life in the Shugo Chara world. Maybe she has to deal with the reactions of her friends and guardians.
Amu Hinamori would be a great character to pair Sakura with. Amu has her own set of Shugo Chara and a strong personality. Together, they could share their experiences with their Shugo Chara and learn from each other. Their different yet complementary personalities could lead to some really interesting interactions and adventures.