One possible plot is that Harry has a special connection with the basilisk due to his link with Voldemort. Since Voldemort was the one who controlled the basilisk before, Harry might be able to use this connection in a different way. He could show the basilisk a different path, away from the dark magic it was previously involved in, and thus tame it.
Well, it could be that Harry uses his Parseltongue ability to communicate with the basilisk. Maybe he convinces it that he means no harm and that they can co - exist peacefully in Hogwarts. He might offer it some sort of deal, like protecting it from those who would want to kill it just because it's a basilisk.
Maybe Harry uses his unique magic skills to communicate with the dragon. He could start by showing kindness and respect, like offering it food. Then gradually gain the dragon's trust and finally tame it.
Hermione would probably be involved. She's always by Harry's side, and she might be researching ways to help Harry tame the basilisk. Maybe she finds some useful spells or information in the library that gives Harry an edge.
Well, perhaps Harry discovers a basilisk egg in some hidden part of Hogwarts. He decides to hatch it out of curiosity or maybe because he believes it could be used to fight against the real threat in the wizarding world. He secretly takes care of it, using his magic knowledge to create the right environment for the egg to hatch.
In this fanfiction, the Basilisk might be accidentally awakened near a magically - created nuke. The Basilisk's presence causes some sort of magical reaction that starts to set off the nuke. Harry, Hermione, and Ron are then on a race against time. They first have to find a way to calm the Basilisk, which is no easy feat as it's a giant and deadly serpent. Then they need to figure out how to defuse the nuke. This could involve using a combination of Hermione's vast knowledge of magic, Ron's loyalty and courage, and Harry's unique skills as the chosen one.
One possible plot could be that Harry discovers a way to control the Basilisk instead of killing it. Maybe he uses some ancient magic or a special spell. He might keep it hidden in the Chamber of Secrets and use it to protect Hogwarts from other dark forces. For example, when Voldemort sends his Death Eaters, the Basilisk could be a secret weapon that Harry unleashes to fight them off.
A Basilisk Patronus could play a key role in a mystery plot. Suppose there are strange disappearances in Hogwarts, and the character with the Basilisk Patronus discovers that the disappearances are related to an ancient curse involving snakes. Their Patronus could help them communicate with the snakes involved, leading to the discovery of the cause of the disappearances and a way to break the curse.
Perhaps in the fanfiction, the Basilisk has a hidden power that can only be unlocked by Harry's wand. A prophecy states that Harry must use his wand to unlock this power and use it for good. However, there are those who want to use the Basilisk's power for evil. Harry has to race against time to figure out how to use his wand to unlock the power and keep it out of the wrong hands. This would involve a lot of exploration of the Hogwarts secrets and battles against dark forces.
In a 'harry is a basilisk animagus fanfiction', the plot could involve Harry being an accidental basilisk animagus. At first, he's scared of his new form. But as he learns to control it, he uses it to his advantage. For instance, he might use his basilisk form to scare off Death Eaters or to find out secrets in the Forbidden Forest. He would also have to deal with the prejudice that comes with being associated with a basilisk.
One interesting plot could be that the Basilisk, instead of being a mindless monster, has a unique form of intelligence. Harry discovers this when he accidentally enters the Chamber of Secrets again. The Basilisk and Harry form an unlikely alliance. They work together to protect Hogwarts from a new dark force that is emerging, which the Basilisk senses through its connection to the underbelly of the school. Their 'mate - like' bond could be based on a mutual understanding of the importance of Hogwarts and its magic.
Perhaps Harry discovers that he has basilisk blood in him. This could change his abilities and how others view him. He might face discrimination from some in the wizarding world who fear the basilisk's power, while others might see him as a potential weapon against Voldemort. The plot could revolve around Harry coming to terms with this new aspect of himself.