A possible plot could be a misunderstanding between Hime and Hiro. Maybe Hiro sees Hime with another character and misinterprets their relationship. This causes a rift between them. However, as they both try to go about their lives separately, they realize how much they miss each other. They then have to work through their misunderstandings and come back together stronger than ever.
One idea could be that Hime and Hiro have to face a powerful new enemy together. They might discover a hidden power within their relationship that helps them defeat the foe. For example, a new type of magic that only activates when they hold hands or something like that.
One way to develop their relationship further is to introduce a conflict that they have to resolve as a team. Let's say there is a prophecy that only they can fulfill together. In the process of trying to understand and fulfill this prophecy, they would grow closer. They might start to see each other in a new light, not just as friends or allies, but as potential life partners.
One interesting idea is that Hiro and Riza are sent back in time in the Princess Resurrection world. They have to adapt to the old ways and also find a way back to their own time. During this time - travel adventure, they might meet younger versions of the characters they know. This could create some really interesting dynamics in their relationship as they learn more about each other's pasts and the history of the world they are in.
Well, a possible plot for a Baymax x Hiro fanfic could be that they discover a new technology that is threatening the city. They have to work together, with Baymax using his medical knowledge and Hiro his technological skills, to stop the threat. It could also be interesting if there was a character from Hiro's past who returns and tries to drive a wedge between him and Baymax.
One great fanfic could be about Hiro and Riza's adventure in the world of Princess Resurrection. Maybe they go on a quest to save the princess together. Their relationship could develop as they face various challenges, like fighting off monsters. They might start as friends and slowly realize their feelings for each other during the journey.
One idea could be Ichigo having to face new and more powerful enemies in his resurrected form. Maybe he discovers new powers that come with this resurrection and has to learn to control them quickly to save his friends.
Perhaps a character who was thought to be dead returns with a new mission and tries to recruit the Young Justice team for a greater cause. This could lead to internal conflicts within the group as they decide whether to trust this new/old ally.
One idea could be a time - travel plot. Marceline accidentally sends herself and Princess Bubblegum back in time to when Ooo was first being formed. They have to work together to find a way back to their own time while also dealing with the primitive versions of their friends. Another could be a body - swap story. They wake up one day in each other's bodies and have to navigate each other's lives for a day, which leads to a lot of humorous and heartwarming moments as they understand each other better. A third plot could be a mystery where something strange is happening in the Candy Kingdom and Marceline teams up with Princess Bubblegum to solve it, and during the process, they discover hidden feelings for each other.
One idea could be that Logan is a commoner who discovers he has a special power that is connected to the princess in Fable 3. They meet by chance when the princess is on a quest to find a solution to a dark magic that is plaguing the kingdom. Logan uses his power to save the princess from a dangerous creature, and from there, a bond begins to form between them as they work together to save the kingdom.
Well, perhaps the reader is an artist who has a special ability to paint the night sky in a way that seems to call out to Rosalina. She becomes intrigued and visits the reader's world. The two then start to collaborate on creating beautiful art that combines the magic of Rosalina's realm and the creativity of the reader. Or, the reader could be a scientist studying comets and Rosalina has some knowledge about a particular comet that could change the reader's research completely, leading to a journey of discovery together.
One interesting idea could be Naruto coming back with new and enhanced jutsu. Maybe he returns with a power that combines all the elements in a unique way.