How about a fanfic where Riki and Rin swap personalities? It would be interesting to see how Rin would handle Riki's more timid and introverted side, and how Riki would deal with Rin's energetic and sometimes brash nature.
A cool idea could be a time - travel fanfic. Riki and Rin find themselves sent back in time to when the Little Busters first formed. They have to deal with the knowledge they already have while trying not to disrupt the past too much.
The emotional depth in these fanfictions can be really engaging. Since Riki has his own insecurities and Rin has her own issues, fanfic authors can really dig deep into how they help each other grow. Some fanfics show Riki becoming more confident because of Rin's straightforwardness, and Rin learning to be more open because of Riki's gentle nature.
In some fanfictions, a plot twist could be that Riki loses his memory related to Rin. This creates a lot of angst and confusion. Rin has to work hard to make Riki remember their relationship, and during this process, they both grow and change in unexpected ways.
In many Little Busters fanfictions, Rin and Riki often have a very special relationship. Riki usually plays a role of support for Rin. He tries to understand her complex personality. For example, Rin's shy and sometimes wild nature is something Riki has to navigate through. Their relationship can be seen as a deep friendship with potential romantic undertones. Riki's patience and Rin's gradually opening up to him make their interactions in fanfictions really interesting.
One idea could be that Riki and Kurugaya go on a secret adventure outside of the usual Little Busters' activities. They discover an old, forgotten place in town that holds a special meaning for Kurugaya's past.
One idea could be that the little pigs and the wolf form an unlikely alliance. Maybe there's a bigger threat in the forest that forces them to work together. For example, a group of hunters arrives and starts causing trouble for all the forest animals. The pigs, with their building skills, and the wolf, with his strength and knowledge of the forest, team up to drive the hunters away.
Perhaps the wolf is under a spell. Little Red, instead of being afraid, tries to find a way to break the spell. She goes on a quest through the forest, meeting different magical creatures along the way who give her clues on how to save the wolf. This turns the story into an adventure - filled journey of redemption for the wolf.
One idea could be Stuart transforming into a bird. He could use his newfound ability to fly and explore the world from above, encountering other bird species and having adventures in the sky. Another could be him turning into a fish. He'd swim in the pond or aquarium, make friends with other fish, and discover the underwater world's secrets.
One idea could be that the stars are actually sentient beings. The little star in the song could be a young star who is on a journey to find its place in the galaxy. It meets other celestial objects like comets and planets that teach it different lessons about existence.
A more complex idea could be that there's an alternate universe where the villains from both worlds combine forces. The Green Goblin teams up with Discord. In this fanfiction, Spiderman and the Mane 6 have to put aside their differences in fighting styles and cultures to defeat this powerful alliance. They learn from each other's strengths, with the ponies teaching Spiderman about the magic of friendship and Spiderman showing them some cool combat moves.
A fanfiction could focus on their relationships. For example, how Sango and Miroku's relationship develops further with Kagome and Rin as witnesses. Maybe Kagome gives Sango some advice on dealing with Miroku's flirtatious nature, and Rin looks up to them as a couple, learning about love and relationships from them.