One great 'Harry and Cedric friends' fanfiction could be 'The Bond Beyond Houses'. It delves into how their friendship might have developed further if Cedric hadn't met his untimely death. It shows them sharing secrets about their respective families and helping each other with their studies at Hogwarts.
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 interesting plotline could be Harry and Cedric teaming up to solve a mystery within Hogwarts that the teachers are oblivious to. They use their combined skills, Harry's courage and Cedric's intelligence, to figure out what's really going on. For example, they might discover a hidden chamber that has been long forgotten and holds a powerful magical artifact. Another could be that they form a study group which then turns into a deep friendship. They help each other with their studies, especially in difficult subjects like Potions and Transfiguration. And finally, they could be united in trying to improve the relations between the different houses in Hogwarts. By being friends, they show that Gryffindors and Hufflepuffs can work together and be allies, rather than just competitors in the Tri - Wizard Tournament.
Their friendship could make the Tri - Wizard Tournament less cutthroat between them. Instead of seeing each other as just competitors, they might help each other out in the tasks. For example, if one of them discovers a useful spell or strategy, they could share it with the other. This would be a big change from the traditional view of the tournament where everyone is out for themselves.
Some fanfictions might explore their relationship through time - travel. For example, if Cedric could go back in time and help Harry in his earlier years at Hogwarts. This would not only change the course of some events but also show a different side of their friendship, as Cedric would be the more experienced one guiding Harry.
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.
There's a fanfiction called 'Cedric's Unfinished Story'. In this one, it explores what could have happened if Cedric had survived the Triwizard Tournament. It's really interesting as it weaves new adventures for Cedric, involving him in the fight against Voldemort in a different way.