It could be related to a prophecy. The character was fated to be immune to poison for a greater purpose, like saving the wizarding world from a poison - based dark magic plot. So, they play a crucial role in the story without the fear of being poisoned like others.
It could be a Death Eater in disguise. They might be trying to get rid of Harry once and for all to please Voldemort. They could have slipped the poison into his food or drink when he least expected it.
Yes, in some fanfictions, Harry Potter is made immune to the Veela's charm. It could be due to some kind of magic protection he has. Maybe Dumbledore cast a spell on him early on to protect him from all kinds of powerful magical influences, including the Veela's allure.
One implication is that he becomes a more formidable opponent in battles. Enemies who rely on poison - tipped weapons or poison - based jutsus would be at a disadvantage against him.
In certain fanfictions, Naruto's immunity to poison might be a result of his training with a hermit or some ancient sage. They could have taught him special techniques or given him elixirs that made his body resistant to poison over time. For example, he might have had to endure harsh trials in a poisonous environment, and his body gradually adapted to it, leading to complete immunity.
One trait Harry might inherit is a strong sense of self - preservation. Poison Ivy is always looking out for herself in her own world, and Harry could have that same instinct. This could make him more cautious in dangerous situations. Also, he might inherit her ability to blend in with her surroundings. Since Poison Ivy can use plants to camouflage herself, Harry could have a natural knack for hiding or remaining undetected in the wizarding world. Maybe he could use magic in combination with this ability to stay out of harm's way more effectively.
In certain fanfiction, Harry Potter is immune to Veela allure. This could be due to a magical protection he unknowingly has. Maybe it was a side - effect of the love magic that protected him from Voldemort as a baby. This protection extends to other forms of magical seduction, including the Veela allure, and it is often shown when Veela characters are around and others are affected but Harry remains unfazed.
One theme could be the challenges of dealing with an unexpected and unorthodox situation in the wizarding world. It might explore how Harry, being a male, copes with pregnancy in a world where such a thing is not the norm for his gender. Another theme could be the reactions of his friends and the wizarding community at large.
In Harry Potter fanfiction, dreams can be used as a foreshadowing device. For example, a character might dream of a future event that later comes true in the fan - created story. Realities, on the other hand, are often distorted. Fanfiction writers may change the existing reality of the Harry Potter world, like creating new spells or altering the relationships between characters. This exploration creates a new and interesting take on the beloved wizarding world.
Perhaps Dumbledore, in his infinite wisdom and his knowledge of some ancient magic, sends Harry to the Pokemon world for a special training. Harry would have to adapt quickly. He would find that some of the values in the Pokemon world, like the friendship between trainers and Pokemon, are similar to the loyalty he values in his friends at Hogwarts. He could learn new combat strategies from Pokemon battles that he could potentially bring back to his fight against Voldemort in the wizarding world.
The fanfiction could develop Harry's leadership side further. He has to lead the workers on the Quidditch farm, so he would become more confident in giving orders and making decisions related to the farm's operations. Also, his love for Quidditch would be expressed in a new way, not just as a player but as someone responsible for all aspects of the Quidditch - related activities on the farm.