It challenges the lore by blurring the line between the wizarding and Muggle worlds. In the original, there was a clear separation, and wizards were generally cautious about interacting too closely with Muggles. But in this fanfiction, Harry is dating a Muggle, which goes against that separation.
They could be brought in as allies or mentors. For instance, Dumbledore could be a wise figure guiding the Tenno, sharing his knowledge of magic and ethics. His wisdom could help the Tenno make difficult decisions in their battles against the various factions in the Warframe universe.
One way to explain it could be through the influence of a powerful dark object. Maybe he stumbles upon an ancient and cursed artifact that slowly corrupts his mind. It starts to make him see the world in a different, more self - serving way, and eventually turns him into a villain.
In fanfiction, wandlore often goes beyond what J.K. Rowling established. For example, in the official lore, we know a bit about the core materials like phoenix feather, but fanfiction might introduce new and exotic core materials. In fanfiction, wands can also have more complex relationships with their owners. While in the official lore it's somewhat basic, in fanfiction a wand could have a 'personality' of its own and be more actively involved in a wizard's life.
The characters could be travelers from another dimension. For instance, Harry could arrive in Tamriel through a mysterious portal. His skills as a wizard could be seen as a rare and powerful form of magic in the Elder Scrolls world. He might be sought after by different factions, either for his knowledge or for his power.
The Lich King has a lot of dark and powerful magic. In fanfiction, characters like Dumbledore might be intrigued by it. He could study it to find ways to counter Voldemort. Maybe he tries to understand the nature of the Lich King's control over the undead to see if there are any parallels with Voldemort's control over his Death Eaters.
In certain fanfictions, Harry is immune to Veela. Well, it might be part of the author's creative take on his character. The author might think that Harry, being the hero, should have some unique abilities that set him apart. So they give him this immunity. It could also be related to his experience with facing various dark magic. His exposure to different forms of magic has perhaps made his magical defenses strong enough to resist the Veela's allure.
Yes, there are some out there. Some fanfictions explore this dark concept in a way that stays true to the overall magical world JK Rowling created. They might have complex reasons for Harry to take such an extreme action, like being under the influence of dark magic or a misunderstanding that twists his perception of Dumbledore.
In fanfiction, the author often has the creative freedom to play with the rules. So, it could be that there is a new form of magic introduced that is unique to the Azkaban setting in this story. This new magic might have affected Harry in such a way that he becomes pregnant. It could also be related to the strange and often malevolent energy that surrounds Azkaban, which has some sort of mutagenic effect on him.