One great Hufflepuff and Slytherin love fanfic is 'The Unlikely Bond'. It beautifully explores the relationship between a kind - hearted Hufflepuff and a somewhat misunderstood Slytherin. The characters' growth throughout the story as they fall in love is really engaging.
Growth is also a prevalent theme. Both characters grow as individuals within the relationship. The Slytherin may become more empathetic, influenced by the Hufflepuff's gentle nature, and the Hufflepuff may gain more confidence, inspired by the Slytherin's ambition. Their love story is often intertwined with their personal growth.
There's also 'The Badger's Allure for the Serpent'. In this fanfic, the Hufflepuff character has a unique charm that attracts the Slytherin. It shows how their different house traits can complement each other in a relationship. The Hufflepuff's loyalty and the Slytherin's ambition create an interesting dynamic.
The contrast between the two houses. Hufflepuffs are known for their loyalty and kindness, while Slytherins are often seen as cunning. When they fall in love in fanfics, it's like two different worlds colliding in a beautiful way.
I really like 'Green and Yellow'. In this fanfiction, the author explores how a Hufflepuff's loyalty and a Slytherin's ambition can work together. It has some really interesting plot twists, especially when they team up for a Hogwarts - wide competition and have to overcome the biases of their fellow students.
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.
You could also try looking on Tumblr. There are fanfic communities there that might have Hufflepuff x Slytherin lesbian stories. Sometimes, fans post excerpts or links to their fics on Tumblr. It might take some digging around, but you could discover some really interesting and unique takes on this pairing.
In a particular fanfic, a Hufflepuff character has a valuable magical item that some evil forces want to steal. The Slytherin main character, despite the typical house rivalries, decides to protect the Hufflepuff. This could be because they see the Hufflepuff's kind - heartedness and realize that the item in their possession is important for the good of the wizarding world. So, the Slytherin uses their intelligence and strategic thinking to keep the Hufflepuff and the item safe from harm.
You can find 'Hufflepuff and Slytherin fanfiction' on Archive of Our Own (AO3). It has a vast collection of fanfictions from various fandoms, including Harry Potter, and you'll surely find some great stories involving Hufflepuffs and Slytherins there.
One common theme is the idea of overcoming differences. Since Hufflepuffs and Slytherins have very different traits, a lot of fanfics focus on how the two characters can look past those differences and form a relationship.