Love against all odds is also a big theme. Given Snape's reputation and his often - gloomy nature, the idea of him falling in love with an OC is an uphill battle, which makes for an interesting story. For example, in some fics, the OC has to overcome the prejudice of the Hogwarts community towards Snape.
One common theme is identity. Since the OC is Snape's son, they often struggle with their own identity, being related to such a complex character. Another theme is legacy. They have to deal with Snape's actions and reputation.
One common theme is redemption. Since Snape has a rather complex and often dark past in the original series, many fanfictions use the relationship with the OC to show his journey towards redemption. For example, the OC might be the one to make him see the good in himself.
Often, there's the theme of redemption. The OC might be involved in Snape's journey towards redemption. His dominant nature drives the story as he tries to right his wrongs with the OC's help or influence. Also, the exploration of the Slytherin values is a theme. Snape, as a Slytherin, passes on these values in a dominant manner to the OC, who may or may not be from Slytherin.
One common theme is Snape's relationship with the students, especially showing his softer side towards them. For example, he might secretly help a student in trouble in a really cute way.
One common theme could be the exploration of Severus Snape's character in a very different situation. It might also involve how the other characters in the Harry Potter universe react to such an unexpected event.
One common theme is redemption. Many fanfics explore Snape's journey of redemption through his interactions with Hermione. Another theme is the bridging of the generation gap. Hermione represents a new generation of wizards, and Snape is from an older one, and their relationship can symbolize the mending of differences between the two. Also, there's often the theme of mentorship, with Snape teaching Hermione in some way.
Another theme is friendship or an unlikely alliance. Since they are two very different characters in the original Potter universe, fanfics like to play with the idea of them coming together for a common cause. This could be to protect Hogwarts or to fight against a new dark threat. It makes for interesting character dynamics as they learn to work with each other.
One common theme is redemption. Snape is often depicted as seeking redemption for his past actions towards Harry. Another theme is the discovery of hidden truths. Harry might find out things about Snape that change his view of him completely. Also, the theme of an unlikely friendship or bond is very common. Despite their differences, they form a connection in many fanfics.
Another theme is the exploration of their relationship. Since they had a rather antagonistic relationship in the original books, fanfic often delves into how they could have come to understand each other better. This could involve things like Harry realizing Snape's true motives earlier or Snape being more open with Harry about his past and feelings.
One common theme is friendship. Since there are three OCs, their relationships with each other often form a big part of the story, like in 'The Trio's Friendship' where they rely on each other through thick and thin.