Well, one possible plot could be that they start off as rivals in a school competition but gradually discover their feelings for each other. Maybe they bond over a shared love for a particular magical subject or a secret place in their wizarding school.
Harry might still be brave and impulsive as in the original story. But in this fanfiction, his relationship with Cedric could make him more open and vulnerable. He would be more likely to share his thoughts and fears with Cedric.
Well, in some fanfictions, Cedric might use his magical skills to shield Harry from physical attacks. For example, he could cast protective spells like Protego Maxima to create a strong shield around Harry when they are in danger. Maybe in a situation where dark wizards are attacking, Cedric steps in front of Harry and uses his wand to fend off the spells aimed at Harry.
One common scenario could be in a dangerous situation at Hogwarts, like when there's a dark magic threat. Cedric might step in front of Harry to shield him. Another could be during a Quidditch match where an opposing team is playing dirty, and Cedric ensures Harry is not targeted unfairly.
It could be Draco Malfoy. In some fanfictions, they put aside their differences and develop a relationship. Sirius would be very surprised as he used to see Draco as an enemy. But seeing Draco's growth and his love for Harry could change Sirius' view.
Well, it could change the whole dynamic. Cedric might protect Harry more, like when facing the challenges at Hogwarts. He could also give Harry advice on dealing with school and the wizarding world, which would make Harry more confident.
One common theme is friendship. Since they were both Hogwarts students, many fanfictions explore how they could have been better friends. Another theme is rivalry turned into respect. Given their initial competition in the Triwizard Tournament, a lot of stories show how they grew to respect each other. Also, the theme of sacrifice often comes up, especially related to Cedric's death and how it affected Harry.
Friendship is a very common theme. Given their initial rivalry in the Triwizard Tournament, many fanfictions explore how they move past that and become true friends. Another theme is mutual respect. They both have their own strengths and in fanfictions, they often come to respect each other deeply.