Well, there's a fanfiction where the brother discovers a prophecy about Harry much earlier than Dumbledore. In order to protect Harry, he starts gathering allies from different magical communities. He teams up with the centaurs, the giants, and some of the more reclusive wizarding families. Together, they form a sort of secret protection squad for Harry, which helps him not only survive but also grow stronger in the face of Voldemort's threats.
A possible example is when a magical animal dies. Let's say a hippogriff that was friendly to Harry is killed. Its death could release a magic that alerts other magical creatures to protect Harry. Or perhaps the death of an old, powerful witch who had a soft spot for Harry. She could have set up a post - death spell that would protect Harry whenever he was in great danger.
The protective brother can change the plot significantly. For instance, if he's highly skilled in magic, he can prevent Harry from getting into some of the dangerous situations he faces in the original series. He might be able to stop Harry from entering the Triwizard Tournament by interfering with the Goblet of Fire's magic. This would then change the entire course of that year at Hogwarts and Harry's relationship with Cedric Diggory and others.
In some fanfics, Harry might protect his younger sibling from bullies at Hogwarts. He could use his magic skills or his status as 'The Boy Who Lived' to intimidate those who try to harm his brother or sister.
One example could be when someone tries to bully Harry, and Edward steps in immediately to stop it.
One example could be a fanfiction where Harry Potter's older sister is a powerful witch who has been secretly protecting Harry all along. She might use her advanced magical skills to thwart Voldemort's plans in ways that the original story didn't explore. For instance, she could have been working behind the scenes to gather information on the Death Eaters and pass it on to the Order of the Phoenix.
One example could be in a fanfiction where there is a new dark wizard threat at Hogwarts. Harry always makes sure Hermione is with him or in a safe place when they suspect danger. He might stand in front of her when the dark wizard's minions approach. Another example is when Hermione is researching a dangerous spell. Harry tries to limit her exposure to it, worried that it could backfire on her.
In certain fanfictions, Harry is shown as being protective of Teddy during magical training. He would be extra careful and supervise Teddy closely to prevent any accidental magic from going wrong and harming Teddy. Also, when it comes to Teddy's emotional well - being, Harry might be the first one to notice if Teddy is sad or troubled. For instance, if Teddy is bullied at school, Harry would take immediate action to stop it and comfort Teddy.
One good example could be a story where a character in the Harry Potter universe, perhaps a minor wizard or witch, is born with a heart disease and has to struggle with it while also dealing with the magical world's challenges. They might face discrimination from some pure - blood families who think a weak body means a weak magic ability. Another could be a story where a spell gone wrong causes a heart disease - like condition in a Hogwarts student, and they have to find a magical cure.
There could be a fanfiction where Ron is jealous of Harry's fame again and acts out in a way that almost ruins their friendship. However, Hermione helps Ron see the error of his ways. Harry, seeing Ron's remorse, forgives him. This then leads to a series of adventures where the three of them face new challenges, with their friendship on a more solid footing because of the forgiveness.
There might be a fanfic in which a master Occlumens is a student at Hogwarts. They are secretly protecting the school from dark forces that are trying to use Legilimency to gain access to certain secrets. This can lead to interesting interactions with the professors and other students, as they try to figure out who this mysterious Occlumens is.