Well, a frequent theme is the clash and then the merging of cultures. The wizarding culture of Harry Potter and the Asian - inspired cultures in Avatar (where Zuko is from) can create a really interesting backdrop. Also, the theme of redemption is often seen. Zuko's own journey of redemption in Avatar can be paralleled or intertwined with characters in the Harry Potter universe who might also be seeking redemption.
Yes, there are quite a few. You can find them on popular fanfiction websites like Archive of Our Own. Some of them blend the magical world of Harry Potter with the elemental bending world of Avatar where Zuko is from in really creative ways, like having characters from both universes interact in unique storylines.
One common theme is the clash of different magical natures. The structured magic of the Harry Potter world against the more seductive and dangerous powers of a succubus. Another could be the idea of temptation, as succubi are often associated with that.
Well, there's often the theme of understanding. Percy, with his rather rigid view of the world, can learn to understand Harry's more unorthodox ways. And Harry can understand Percy's need for order and achievement. Another theme is loyalty. They can show loyalty to each other in the face of various challenges in the wizarding world, be it from dark wizards or from the corrupt parts of the Ministry of Magic.
One common theme is the clash of personalities. Snape is often portrayed as brooding and secretive, while McGonagall is more straightforward. Their interactions can be full of tension and underlying respect.
One common theme is the clash of different power systems. In 'Owari no Seraph', there are vampire powers and human weapons, while in 'Harry Potter' there's magic. Fanfictions often explore how these different powers interact. Another theme is the idea of survival in a new and dangerous world. Characters from one series suddenly find themselves in the other's world and have to figure out how to stay alive.
One common theme is rivalry turned into something else. Since they are from different houses and often opponents in Quidditch, many fanfictions explore how that initial rivalry can evolve. Another theme is mutual respect. As they get to know each other better, they start to respect the qualities in each other that they didn't notice before.
One common theme is the clash of two different worlds - the wizarding world and the vampire world. Another could be the idea of forbidden love, as vampires are often seen as outsiders in the Harry Potter universe. Also, power dynamics are often explored, like who is more powerful between a powerful wizard and a female vampire.
One common theme is family. Sirius is Harry's godfather, so many fanfictions explore the idea of family that goes beyond blood. Another theme is redemption. Sirius has a past full of mistakes and he often tries to redeem himself through his relationship with Harry. Also, protection is a big theme. Sirius is always trying to protect Harry from the threats in the wizarding world.
Well, a lot of 'Harry Potter x Astoria Greengrass' fanfictions have the theme of redemption. Astoria might be seen as a character who can redeem Harry in some ways, or vice versa. Also, the theme of breaking social norms is quite common. Their relationship goes against what might be expected in the wizarding society. Additionally, the theme of discovery is there. They discover new things about each other and about themselves through their relationship.
One common theme is the exploration of power. Since both Harry Potter and Infamous deal with different forms of power (magic in Potter and various powers in Infamous), fanfictions often explore how the fem Harry Potter character manages and combines these powers. Another theme is identity. She has to figure out who she is in this new, combined world.