Well, he'd probably become super powerful. The symbiote could enhance his magic skills and give him some cool physical abilities like super strength and agility. Maybe he could use it to fight the Dark Arts in a whole new way.
I'm not aware of any such fanfic. Fanfiction often contains a wide variety of strange and inappropriate concepts, but this one seems rather extreme and goes against the nature of the original Harry Potter story in terms of the characters' relationships.
A plot could involve a symbiote attaching itself to Harry. This symbiote might have its own motives. It could be from another dimension where magic and symbiotes are intertwined. The symbiote could start influencing Harry's actions, making him more aggressive in his fight against Voldemort. But at the same time, his friends are suspicious of the changes in him. As the story progresses, Harry has to learn to control the symbiote and use its power for good while also dealing with the internal struggle of not losing himself to it.
As a girl, Harry might be more cautious in some situations. In the original story, Harry was often quite bold and impulsive. But now, she might think twice before jumping into dangerous situations. Also, her relationships will shape her differently. She may be more nurturing towards her friends, and this could change the group dynamic. For instance, instead of leading the charge all the time, she could be more of a behind - the - scenes organizer, using her new - found femininity to bring the group together in different ways.
In a fanfic where Harry Potter gets a lawyer, it could lead to some really interesting plot developments. Maybe the lawyer helps him deal with the legal mess left by the Ministry of Magic's previous actions against him. For example, clearing his name regarding the false accusations of being a dark wizard or dealing with property rights issues related to his family's estate at Godric's Hollow.
It's inappropriate because it takes a beloved and well - established fictional world and character and twists it into something completely untrue to its essence. Harry Potter is about magic and heroism, not such adult - themed and out - of - place ideas.
Since Harry Potter is a well - known fictional character from a series mainly targeted at a wide range of audiences including children and young adults, this 'fanfic' idea is inappropriate. It's not in line with the values and the magical world that has been created in the books. There's no real place for such a concept in the legitimate exploration of the Harry Potter universe.
It would be a really strange and exciting mix. Harry could have the Goa'uld's ability to possess others on top of his magical powers. He might use this to his advantage in battles against Voldemort, for example, by possessing Death Eaters. It would also change his relationships with his friends as they might be a bit scared or in awe of this new power.