It could be an unknown character who has been in hiding all this time. Maybe she was once a powerful witch who made a pact with death and became its representative in the wizarding world.
One idea could be that the Goddess of Death has a special connection with Harry. For example, she might appear to him in a time of great danger and offer him a unique power that no one else has. This power could be related to seeing the true nature of dark magic, which would help him in his fight against Voldemort.
One idea could be that Fem Harry, as the Goddess of Death, has the power to see the final moments of every being. She uses this to prevent unjust deaths in the wizarding world. For example, she might foresee a student being wrongly accused and killed by a dark wizard and intervenes just in time. Another plot could involve her dealing with the afterlife. Maybe she discovers that some souls are being trapped and tortured by dark magic in the in - between realm, and she has to fight against the dark forces there to free them.
The goddess could also be an original creation for the fanfiction. A being with a unique set of powers like the ability to control time and space. This goddess could be betrothed to Harry to stop an impending doom that threatens both the wizarding and Muggle worlds. Her powers would be so great that she needs Harry's connection to the wizarding world to fully utilize them and save the day.
It could be Athena. Athena is known for her wisdom in Greek mythology. She could pass on her wisdom to Harry, making him not just a powerful wizard but also a very wise one. He could use this wisdom to solve problems in the wizarding world in a more strategic way.
The goddess could be an ancient Celtic deity. In the fanfiction, the author might choose a Celtic goddess because of the rich magical lore associated with them. For example, Morgan le Fay from Arthurian legend could be reimagined as a goddess in this context, and she could adopt Harry due to her interest in powerful magic users.
She could be an ancient Celtic goddess, like Brigid. Brigid is associated with magic, healing, and smithcraft in Celtic mythology. In the fanfiction, she might see Harry's potential and decide to train him in her unique magical arts.
Yes, there are likely fanfictions out there with this concept. The 'Harry Potter' fandom is vast and creative, and many fans like to explore unique and alternative storylines. Some might imagine Harry having a special connection to the idea of 'Lord Death' in a fan - created narrative.
One possible goddess could be Athena. In the context of the fanfiction, she could bring her wisdom and strategic skills to help Harry Potter. Another could be Aphrodite, adding an element of love and charm to the story, perhaps using her powers to influence the emotions of characters for good or bad depending on the plot.