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.
Maybe a Stargate team comes across an ancient artifact that has a connection to the Highlander's immortality. This could lead to a race between the team and some villains to unlock the secrets of the artifact, with the Highlander in the middle trying to protect it from being misused.
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 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.
One idea could be that the Stargate team discovers a world where the Sentinel abilities are a natural part of the population. The Sentinels' powers could be used to help the Stargate team in their exploration and battles against alien threats. For example, their enhanced senses could detect hidden Goa'uld symbiotes or hidden traps in ancient ruins.
One idea could be to cross Stargate Universe with Stargate SG - 1. Maybe the Destiny crew discovers an ancient technology that was originally developed by the SG - 1 team on Earth. This could lead to a joint mission where both crews have to work together to unlock the secrets of the technology and use it to save both their ships and Earth from a new threat.
Maybe a character from Stargate, say Daniel Jackson, gets stranded in the Farscape universe. He could use his knowledge of ancient cultures and languages to help the Farscape crew with some of their own mysteries. And in return, the Farscape crew could teach him about their unique technology and ways of survival in a very different and often hostile galaxy.
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.
Maybe there's a race in the Stargate universe that has developed a technology similar to the Borg in Star Trek. The Starfleet and Stargate teams come together to study this new threat. They could exchange knowledge about artificial intelligence and combat strategies. For example, the Stargate team might introduce the concept of Ancient technology, which could be used to counter the Borg - like threat in ways that Starfleet has never considered.
A common storyline is the clash of cultures. The military - oriented Stargate teams with their high - tech weapons and strategies meet the lone - wolf immortals of Highlander. There could be a situation where an immortal Highlander is mistaken for an enemy by the Stargate team. Or, the Stargate team might need the help of an immortal to defeat a powerful alien foe that is immune to their normal weapons. The immortals' ability to sense each other could also play an interesting role in a Stargate - Highlander crossover, like when they are on a planet full of hidden threats and the immortal's sense helps the team avoid danger.
Well, in Roswell Stargate crossover fanfiction, there might be a plot where the characters from Roswell get involved in Stargate missions. Imagine the Roswell gang having to deal with the Goa'uld or the Replicators. It could also be about how the technology from Stargate affects the lives of the Roswell aliens. For instance, the Stargate could be a way for them to find a cure for their alien - related problems or a means to enhance their powers.
One idea could be that the Spartans from Halo are sent through a Stargate to an unknown planet in the Stargate universe. They have to team up with the SG teams to fight off a common enemy like the Flood that has somehow spread to that planet. Another could be a scenario where the technology from both universes is combined. For example, the Covenant's energy shields are integrated with the Stargate's shield technology.