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.
Perhaps a plot where Miki discovers a hidden side of Kiseki. Kiseki seems to be acting strange, and Miki, being curious, decides to find out what's going on. It turns out Kiseki is trying to protect a secret place full of beautiful but endangered magical art. Miki then joins Kiseki in his mission to safeguard this place. Along the way, they encounter obstacles and villains who want to destroy the art, but together they manage to overcome them.
Amu could be a great addition. She has a strong connection to the Shugo Chara world and her presence could add more depth to the story. For example, she could be the one who gives Miki and Kiseki advice when they face difficulties in their relationship or in their adventures.
One plot idea could be that Miki and Kiseki get lost in a mysterious dream world together. They have to work as a team to find their way out while facing various challenges that test their friendship. For example, they might encounter strange creatures that can only be defeated by combining Miki's artistic skills and Kiseki's logical thinking.
Their relationship could be developed uniquely by having them switch powers for a short time. This would force them to understand each other on a deeper level as they experience the world from the other's perspective. For instance, Miki would have to deal with Kiseki's more logical and structured way of using powers, while Kiseki would have to adapt to Miki's more creative and fluid approach.
A more adventurous fanfiction could involve Yoru and Miki going on a journey through different dimensions in search of a special crystal that has the power to grant any wish. Along the way, they encounter strange creatures and have to rely on each other's unique abilities to survive and reach their goal. Their relationship evolves from comrades to something more romantic as they face numerous perils together.
Some anime - related forums might also be a good place to find fanfiction. Members often share their own works or recommend good ones they've read. For example, there could be a dedicated Shugo Chara forum where fans discuss all aspects of the series, including fanfiction about Yoru and Miki.
A plotline could be that Amu and Tadase get transported to a parallel universe where their roles are reversed. Amu is now the cool and confident one while Tadase is the more shy and reserved. This would create an interesting dynamic as they have to adapt to these new personalities and still find a way to get back to their original world.
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.
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.
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.
One idea could be exploring the backstories of the Guardian Characters in more depth. For example, how did they come to be and what were their experiences before they became Guardian Characters.