Fanfiction often takes liberties with the relationship between Izuku and Toga. It could also involve them teaming up against a common enemy. This would require them to put aside their differences and work together. Writers can use this to create exciting action - packed scenes and explore how they would communicate and cooperate in such a situation.
Toga is still alive in the manga. The creators haven't decided to end her character's journey with death. Usually, such major plot points are well-signaled and haven't happened for Toga so far.
One possible plot could be that Ochaco wakes up one day as a kid and has to figure out how to reverse it while also dealing with her now child - like body. She might struggle with her hero training as a kid, like not being able to reach high places or lift heavy objects like before. And she could have cute interactions with her classmates who are now much bigger and older - looking compared to her.
Maybe Toga uses his powerful abilities. He could have some special powers like super strength or the ability to manipulate time or space. With these, he might be able to stop Kikyo from harming Kagome.
In fanfiction, their relationship could develop through a common enemy. Suppose a new, powerful villain emerges who threatens both Toga's goals as a villain (in her own twisted way) and Izuku's mission to protect. They might be forced to team up against this foe. During their time together, they would learn about each other's strengths and weaknesses, and this could lead to a deeper understanding and a change in their relationship.
Their relationship could start with mutual self - interest. They both recognize the power and chaos the other can bring. For example, Toga's ability to transform could help Villain Deku infiltrate places, and he can offer her protection in return.
Mentally, Ochaco would struggle. She has the mind of an older person but is trapped in a child's body. She would have to deal with the frustration of not being able to communicate as effectively as she used to. And she might feel a sense of loss for her normal life and the progress she had made in her hero training and relationships.