Sure, there is. A lot of fanfiction writers are drawn to the idea of pairing Harry Potter and Bellatrix because of the extreme contrast between them. One story I read had Bellatrix under a spell that made her forget her allegiance to Voldemort, and she ended up helping Harry. This led to a series of events where they started to see each other in a different light, and the fanfiction explored their evolving relationship in great detail.
One common theme is loyalty. Bellatrix's loyalty to Voldemort and Narcissa's more complex loyalty to her family and sometimes self - preservation create interesting dynamics. For example, in many fanfictions, Narcissa will try to protect her family from Bellatrix's more extreme actions.
Redemption is a common theme. Given Narcissa's past as a Death Eater, fanfictions often explore her journey towards redemption, with Harry sometimes being the catalyst for that change. Another theme is the power of unexpected alliances. Their relationship, which is not in the original books, represents an alliance that cuts across the divide between the 'good' and the 'bad' in the wizarding world.
Well, a frequent theme is the exploration of good vs. evil in a more nuanced way. Since Harry represents good and Bellatrix is a symbol of evil in the original series. Fanfictions might also focus on the idea of forbidden love, as their relationship goes against the natural order of things in the Potterverse. There could be themes of manipulation too, as Bellatrix is a master of that and how Harry could deal with it if they are in some sort of relationship.
There's also 'Bellatrix, Narcissa and the Unseen Alliances'. This fanfic explores how the two sisters might have formed secret alliances or had different agendas during the wizarding war. It really brings out their individual personalities and how they interact with other characters from the series.
Some fanfictions have a plot where Bellatrix and Narcissa discover a long - lost magical artifact. Their search for it and what they plan to do with it once found can lead to all sorts of interesting interactions with other characters like Harry Potter. For example, they might try to use it against him or it could be a way to redeem themselves.
In some fanfictions, Bellatrix's loyalty to Voldemort is re - examined in the context of her relationship with Narcissa. For instance, Narcissa might try to persuade Bellatrix to question Voldemort's motives, which could lead to a rift in Bellatrix's unwavering devotion. This gives a new angle on Bellatrix's character, making her more complex than just a blindly loyal Death Eater.
Well, a frequent theme is the exploration of family loyalty. Narcissa's loyalty to the Black family and how it might be affected by her relationship with Harry. Also, there's the theme of secrets. In many fanfictions, they share secrets that bind them together. Like in some stories, Narcissa knows something crucial about Harry's past that she has been hiding from the rest of the wizarding world. It adds a layer of mystery and intrigue to their relationship.
One great 'Harry Potter x Bellatrix fanfic' is 'The Unlikely Bond'. It explores a very different relationship dynamic between Harry and Bellatrix. It shows how Harry, in this fictional scenario, tries to understand Bellatrix's twisted mind and in the process, they develop a strange connection that is full of tension and mystery.
Another theme could be redemption. Although Bellatrix is a villain, some fanfictions might explore if there could be any chance of her changing or at least having a moment of self - realization in relation to Harry. It could also be about Harry trying to understand the reasons behind Bellatrix's actions. Well, fanfiction gives a lot of room for such creative interpretations. For instance, the exploration of their family ties in a different light, like how Bellatrix is related to Sirius and how that might affect her and Harry's relationship in an alternate story line.