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.
Perhaps there could be a plot where a powerful artifact from the Highlander universe ends up in Xander's world. Xander, being the resourceful character he is, gets involved in the search for this artifact. Along the way, he meets a Highlander who is also after it for different reasons. They initially clash but then realize they need to work together.
One idea could be a Highlander character finding a Stargate and being transported to different planets. They could use their immortality to survive in the various dangerous situations on alien worlds and perhaps encounter Goa'uld who are interested in their immortality.
A possible plot could involve an immortal villain from the Highlander world using the Stargate network to spread chaos across different planets. The SG1 team, with the help of some friendly Highlander characters, has to track him down and stop him before he causes too much destruction.
One idea could be that Buffy and the Highlander team up to fight a common supernatural enemy that is causing chaos in both their worlds. Maybe a powerful demon that can't be killed easily, and the Highlander's immortality and Buffy's slayer skills are needed to defeat it.
Well, perhaps there's a Highlander who stumbles into the wizarding world. He might be confused at first by all the magic but then realizes he can use his immortality and combat skills to his advantage. He could get involved in the fight against Voldemort, using his unique abilities. Or, there could be a magical tournament in the Harry Potter world that also includes Immortals from the Highlander world, and they compete using both magic and their Highlander powers.
A really cool concept would be if Xander was a Muggle - born in the Potter world but was raised in a different dimension like his own. When he discovers his magical heritage, he brings his different perspective and technology (to the extent that it can work in the wizarding world) to Hogwarts. He could introduce things like improvised weapons made from Muggle items to the Dueling Club, or use his street - smart knowledge to outwit the Slytherins in their schemes. He and Harry would bond over their shared experiences of feeling like outsiders in different ways.
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.
One idea could be a crossover with the 'Buffy the Vampire Slayer' and 'Supernatural' universes. Xander Harris could team up with the Winchester brothers to fight demons that are a threat to both their worlds. Another idea is a crossover with 'Doctor Who'. Xander could find himself on the TARDIS and have adventures across time and space. A third idea might be a crossover with 'Harry Potter'. Xander could visit Hogwarts and help the students there deal with a new kind of dark magic that is related to his experiences in Sunnydale.
Maybe a story where Harry Potter himself is revealed to be an Immortal. He would have to balance his fight against Voldemort with the rules of the Highlander's world. This could add a whole new layer of complexity to his journey. He might learn new combat skills from the Highlander lore and use them against Death Eaters.
One interesting story could be about how the characters from Highlander and SG1 team up to fight a common enemy that has the ability to travel between different dimensions. This would bring together the immortality concept from Highlander and the inter - galactic exploration of SG1.
One plot idea could be that Xander gets transported into the Star Trek universe. He could use his knowledge from the Buffyverse to help the Star Trek crew deal with some supernatural - like threats that they're not used to. For example, a creature that is similar to a demon from Buffy but with a sci - fi twist.