One idea could be that Xander discovers he has the immortality of a Highlander. He then has to navigate his newfound ability while still dealing with the usual supernatural threats in his world. Maybe he teams up with a Highlander character to learn more about his new condition.
Xander and Duncan MacLeod could be a great pairing. Duncan has centuries of experience as a Highlander, and Xander has his own unique experiences dealing with vampires and demons. They could learn a lot from each other.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, including Highlander crossovers. Just use the search function and type in 'Highlander Crossover' to find relevant stories.
Yes, there are likely some interesting ones. You can start by looking on fanfiction websites. Some might explore unique character interactions, like how the characters from Torchwood would interact with the immortals in Highlander. They could create new and exciting storylines that combine the two universes' mythologies.
The uniqueness lies in the combination of two very different universes. NCIS is a modern - day crime - solving show, while Highlander has elements of immortality and a different kind of combat. When crossed over, it can create stories that are full of contrasts and new possibilities.
Yes, there are likely some interesting ones out there. A good place to start looking could be on fanfiction websites like Archive of Our Own. Some might explore how the characters from NCIS interact with those from Highlander in unique crime - solving scenarios or character - development arcs.
Another common theme is the clash of different mythologies. The show Xander is from might have its own set of supernatural rules and creatures, and when combined with the Supernatural world, there are a lot of interesting interactions. This can lead to stories where the characters have to figure out how to deal with new and unexpected threats that come from the combination of these two different mythologies.
One great Xander crossover fanfiction could be one where he crosses over into the 'Buffy the Vampire Slayer' and 'Supernatural' universes. It might have him teaming up with the Winchester brothers to fight vampires that are a bit more complex than the ones he's used to in his own world.
There could be a 'forever highlander crossover fanfiction' that pairs the Highlander with the 'Buffy the Vampire Slayer' world. Imagine a Highlander in Sunnydale, facing vampires and demons. His immortality would give him an edge in battles, and his code of honor might clash or align with Buffy's mission. It would be a great mix of two different supernatural concepts.
You can try websites like Archive of Our Own (AO3). It has a vast collection of fanfiction from various fandoms, and there's a good chance you'll find some torchwood highlander crossover stories there.