A third plotline could be centered around a mystery at Hogwarts. The female Harry Potter and Severus have to put aside their differences and work together to solve it. Along the way, they discover new things about each other and the history of the school. Their relationship evolves from one of distrust to one of trust and admiration as they unravel the mystery.
One possible plotline could be the Hogwarts students being transported to the world of Game of Thrones and having to navigate the political intrigue and power struggles using their magic. For example, Hermione could use her intelligence and knowledge of spells to outwit the scheming lords in Westeros.
A plotline might be centered around a misunderstanding. For instance, Starfire's alien ways could be misinterpreted by Peter or his friends, leading to some initial tension. But as they get to know each other better, they realize they have more in common than they thought. Or perhaps they get transported to an alternate dimension together and have to rely on each other to find their way back home. This would force them to work together closely and develop a bond that goes beyond their initial impressions of each other.
One possible plotline could be that Harry breaks a major Hogwarts rule, like sneaking into the Forbidden Forest. Snape catches him and decides that a severe punishment is in order, which in this case is spanking. Another could be that Harry fails a very important Potions test repeatedly, and Snape, frustrated with his lack of progress, resorts to this extreme form of discipline. However, these are all just fan - created ideas and not part of the official story.
Maybe the plot could involve a common enemy. The female Harry Potter and Klaus have to team up to fight against a powerful dark force. Along the way, they start to understand each other's motives and personalities better. Harry Potter's sense of justice and Klaus' survival instincts could either clash or complement each other. As they face challenges together, a romantic or at least a strong bond of friendship could develop between them.
There are quite a few out there. You can start by looking on fanfiction.net. Search for tags like 'female Harry Potter' and 'Severus Snape'. Some popular ones focus on their relationship in a more platonic or mentor - mentee way, exploring how a female Harry might change the dynamic of their interactions within Hogwarts.
One possible plotline could be the discovery of Harry's true parentage. Maybe there was some sort of magic or spell that was cast to hide the truth. Harry would then have to deal with the shock of finding out that his parents are two people who were enemies in the original story. Another plotline could be how the wizarding world reacts to this revelation. There would likely be a lot of confusion and perhaps even some opposition.
Some fanfictions have a plotline where Snape survives the Battle of Hogwarts and he and Harry have to deal with the aftermath together. They might have to face the wizarding community's reaction to Snape's past actions and also work on rebuilding Hogwarts. There are also stories where Harry, while still at school, starts to understand Snape's difficult position between Dumbledore and Voldemort and tries to help him in some way, which creates a whole new dynamic between them.
A plotline could involve the demonness being an outcast in her own world and finding a new home in the wizarding world of Harry Potter. Harry, being the empathetic hero, helps her adjust. They team up to solve a mystery that is related to both their worlds, like a magical object that has the power to either destroy or save both the demon realm and the wizarding realm. It's a story of acceptance and cooperation.
Perhaps there could be a plot where Gokudera Hayato is on a mission in the wizarding world and encounters the female Harry Potter. They team up to fight a common enemy, which could be a dark wizard from Harry's world or an enemy from Gokudera's world that has infiltrated the wizarding world. As they work together, they learn about each other's cultures and powers and form a bond.
One possible plotline could be that Harry discovers a plot against Snape before it's too late. Maybe he overhears some Death Eaters planning to ambush Snape while he's on a solitary mission. Harry then uses his invisibility cloak and magic skills he's learned over the years to outwit the Death Eaters and save Snape. This would show Harry's growth as a wizard and his ability to set aside past differences with Snape.