One implication is that it changes the moral landscape of the story. Necromancy is a dark art in the Harry Potter universe, so if Harry is a necromancer, he might be seen as a more morally ambiguous character.
If Harry Potter is a powerful necromancer in fanfiction, it can lead to new and complex relationships. Other characters may either fear him or be drawn to his new - found power. Also, it could mean that the power balance in the wizarding world is disrupted. Since necromancy is a very powerful and often feared form of magic, Harry having this ability would put him in a unique position. He might face opposition from those who uphold the traditional view of magic, while also attracting those who want to use his power for their own ends.
The implications are far - reaching. First, it challenges the established magic system in the original series. Harry's necromancy would likely be seen as a threat by the Ministry of Magic. Second, it could change the way his friends interact with him. Hermione, for example, who is very much against the dark arts, would have to reevaluate her relationship with Harry. Third, it could open up new plotlines regarding the origin of necromancy and how Harry came to possess such a power. It might also lead to a conflict with Voldemort, as two powerful forces in the dark arts would be in competition, which is very different from their relationship in the original story.