Well, one possible interesting plot could be the discovery of a new kind of magic related to the poisonous snake. Maybe the snake has a unique power that can either help or harm the wizarding world.
Ron could also be the main character. He has his own courage and loyalty. He could be the one who has an unexpected connection with the poisonous snake, like being able to communicate with it in a special way that no one else can.
There could be many interesting fanfictions involving Harry Potter and a snake. Maybe the snake is a magical creature with special powers that helps Harry in his adventures. For example, it could lead him to hidden secrets or protect him from dark wizards.
In Harry Potter fanfiction, a 'poisonous kiss' could be a creative and often dark or dangerous plot device. It might involve a kiss that has a harmful magical effect, perhaps used by a villain to incapacitate or harm a character. For example, a dark wizard might use a poisonous kiss to put a curse on an unsuspecting victim, like a love spell gone wrong that instead of bringing love, brings harm.
It could also be a snake from the Forbidden Forest. There are many unknown and mysterious creatures in there. A snake from that forest might be more in tune with the natural magic of Hogwarts. Harry, being the curious and kind - hearted person he is, might come across it during one of his adventures in the forest and start a friendship with it.
It could be Nagini. Even though in the original series she was more of a servant to Voldemort, in the fanfiction, she could have a different personality and form a bond with Harry.
The most poisonous snake in the world was the inland taipan. The inland taipan was an Australia snake. It was large, strong, and could secrete lethal venom. The venom released from each bite was enough to kill 100 adults and 500,000 rats. Its toxicity was 20 times that of the king cobra. The venom of the inland taipan snake was a neurotoxin and a cardiac toxin. Generally, after biting a smaller prey, the prey would die before it could let go. However, the inland taipan lived in the dry inland areas of Australia. It was shy and rarely hurt people.
Maybe it's an anonymous gift from someone who has been observing Harry. This person could be a former student of Hogwarts who has a special connection to snakes and sees that Harry has a kind heart and would take good care of the snake. So they leave the snake for Harry to find with a simple note that just says 'For Harry Potter'.
The snake's help could represent an unexpected alliance. In the Harry Potter series, snakes are often seen as symbols of Slytherin, which is usually at odds with Harry. So it's significant as it breaks the stereotype of enmity between Gryffindor (represented by Harry) and Slytherin.
The snake might use its ability to move stealthily. It could spy on the enemies for Harry Potter. For example, it can slither into hidden places where humans can't easily reach and gather important information.