One possible plotline could be about their initial meeting and the forming of their friendship. Maybe they meet during the founding of Hogwarts and discover that they have different ideas about how to run the school, but also find common ground.
You can try fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions on various topics, including those related to the Harry Potter universe and specifically about Helga Hufflepuff and Salazar Slytherin.
Another plotline could involve the two characters facing prejudice from their respective houses or within the wizarding world in general. They then have to fight against this discrimination to be together. This can add a lot of drama and tension to the story. For instance, their friends in their houses might not approve of their relationship at first.
A common plotline involves Hermione unlocking some sort of hidden power or knowledge that was left behind by Salazar Slytherin. For example, she might discover a long - lost spellbook of his and learn some powerful magic from it. Another plotline could be that she is chosen by some ancient Slytherin - related magic to carry out a task that has been long forgotten, which could put her at odds with some of the Hogwarts faculty or her friends initially as they misunderstand her intentions.
Often, Helga Hufflepuff is portrayed as a guiding figure. For example, in some fanfics, her spirit gives advice to fem Harry in dreams. She represents the values of fairness, kindness, and hard - work which fem Harry tries to uphold.
A possible plot twist could be that Harry discovers he has an inheritance from Helga Hufflepuff that gives him the power to control certain magical creatures in a unique way. This could change his adventures in the Forbidden Forest, for instance.
One possible way is through a long - lost relative. There could have been a distant family member who was part of the Slytherin line and through some complex family history, that relation is passed down to Harry.
Well, one way could be through a time - travel spell gone wrong. Maybe Harry accidentally gets sent back in time and is mistaken for a Slytherin heir. Since he has some of Voldemort's powers due to the Horcrux connection, it could be misinterpreted as Slytherin - like magic.
One possibility is that Harry could be initially in awe. Salazar Slytherin is one of the founders of Hogwarts after all. Harry might question Slytherin about the negative reputation his house has acquired over the years. Maybe Slytherin would try to convince Harry of the true values his house was meant to uphold, like ambition and cunning in a more positive light.
Perhaps in a parallel universe, the Hogwarts founders had different relationships. Hermione discovers this parallel world where she and Salazar are destined to be together. She has to fight against the norms of both worlds to be with him. This could involve battles against traditional pure - blood ideas in Slytherin's world and modern prejudices in her own world.
Perhaps the lemon is used in a Hufflepuff - themed spell. Helga Hufflepuff might have created a spell that uses a lemon as a focusing object. Harry Potter, during his adventures, finds a torn page of an old spellbook that mentions this spell. He then has to figure out how to complete the spell with the lemon and in doing so, he uncovers more about Hufflepuff's unique magic. This could involve interactions with Hufflepuff ghosts or descendants who know more about the lemon - related spell.