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.
Itachi is another option. His actions in the Naruto series are often shrouded in mystery. In a fanfic, his Shugo Chara could be a manifestation of his true intentions and the sacrifices he made. The fanfic could then delve deeper into his character, showing how his Shugo Chara influenced his decisions and how he interacted with other characters in the context of the Shugo Chara world.
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.
It depends on the plot and the author's imagination. Maybe it'll be in the next chapter or much later.
A plot could involve Miki and Kiseki competing in an art competition within the Shugo Chara universe. Miki, being an artist - like character, is naturally talented, but Kiseki has some unexpected skills up his sleeve. As they compete, they also start to understand each other better and form a deeper bond. They learn from each other's styles and techniques, and in the end, they might even create a collaborative piece that wows everyone.
There could be other characters who are trying to stop Chara from eating the human. Maybe a group of friends or family of the human, or some sort of protectors in the world of the fanfic. They would play a crucial role in trying to prevent the dark act from happening.
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.
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.
The 'Shugo Chara Rated M Fanfiction' could be about a variety of things. It might take the basic concepts of the 'Shugo Chara' world, such as the idea of the Guardian Characters and the search for self - discovery, and expand on them in a more mature way. For example, it could show how the characters deal with more serious problems like betrayal or loss in a more complex emotional context. Maybe it also delves deeper into the romantic relationships between the characters, exploring aspects that are not fully developed in the original series, but always within the framework of the 'Shugo Chara' universe.
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.
There could be some 'shugo chara rima x nagihiko fanfic' on Tumblr as well. Sometimes fans share their works or recommend others' works there. You can search for relevant tags such as '#shugo chara', '#rima x nagihiko' etc. However, the quality may vary, so you need to read through a bit to find the ones that suit your taste.
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.