Another theme is the idea of unexpected partnerships. Since they are from different universes in the Marvel world, fanfictions like to play with the concept of how they would team up and what kind of dynamic they would have. For example, Spider - Man's scientific knowledge might combine with Lady Sif's warrior skills in some stories. It makes for an exciting read as they both bring unique strengths to the table. And also, there is often a theme of mutual respect growing between them as they learn about each other's abilities and values over the course of the story.
One great Spider - Man and Lady Sif fanfiction is 'The Unlikely Alliance' which shows them teaming up to fight a common enemy in a unique and exciting way.
One idea could be a crossover where Naruto is somehow transported to the realm where Lady Sif is from. They could team up to fight a common enemy. Naruto's unique jutsu and Lady Sif's combat skills would make for an interesting combination.
Another possibility is a story where a spell goes wrong in the Harry Potter world and opens a portal to Asgard. Harry and his friends end up in Asgard and meet Lady Sif. They form an alliance to stop an evil wizard who has also found his way to Asgard and is trying to steal Asgardian magic for his own dark purposes. Lady Sif could teach them some Asgardian magic while they introduce her to things like the Marauder's Map.
Yes, there are some out there. You can start by looking on fanfiction websites like Archive of Our Own. Just search for 'Harry Potter and Lady Sif' and you'll likely find a variety of stories, some might be about them teaming up in a magical adventure, or perhaps a crossover story where the worlds of Harry Potter and Thor (where Lady Sif is from) collide in an interesting way.
Obviously, Naruto and Lady Sif are the main characters. Naruto is a ninja from the Hidden Leaf Village, known for his determination and powerful jutsu. Lady Sif is an Asgardian warrior, strong and skilled in combat.
A good plot could be that a powerful villain is threatening both Naruto's world and Lady Sif's realm. They are brought together by a magical or technological device. They initially don't trust each other due to their different ways, but as they start to understand each other's powers and motives, they join forces to defeat the villain.
They could interact in a way that combines their different skills. For example, Percy could use his water - based powers to help Lady Sif in situations where she needs to cross large bodies of water or deal with water - based enemies. Lady Sif could share her knowledge of Norse magic and weaponry with Percy. Their interactions could also be filled with cultural exchanges. Percy could introduce Lady Sif to things like blue food from Camp Half - Blood, while she could tell him stories of the great battles in Asgard. This would create a unique bond between them as they learn from each other.