One idea could be that the characters from Torchwood and Arrow team up to fight a common enemy that has some sort of interdimensional powers. Maybe a villain from Arrow's world has found a way to access Torchwood's technology and is using it for evil, and the two groups have to combine their skills to stop it.
Captain Jack Harkness from Torchwood and Oliver Queen from Arrow could have some really interesting interactions. Jack's flamboyant and long - lived nature contrasted with Oliver's more brooding and serious personality would create a lot of tension and also some great comedic moments. They could both learn from each other's leadership styles.
Arrow crossover fanfiction is fan - created stories that involve characters from the 'Arrow' series crossing over with characters from other shows, universes, or even different timelines within the Arrowverse. It allows fans to explore new storylines and interactions that may not have occurred in the official canon.
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.
One great piece of 'Supernatural' and 'Torchwood' crossover fanfiction is 'The Convergence'. It weaves together the two universes really well, with the characters from both shows interacting in a very believable way. The story has a great plot that involves a common threat that both the Winchester brothers from 'Supernatural' and the Torchwood team need to face. It's full of action and also some really interesting character moments.
One common theme is the clash of different kinds of supernatural or otherworldly elements. Since 'Supernatural' has a lot of demons, angels, and ghosts, and 'Torchwood' deals with aliens and time - space anomalies, when they cross over, there's often a struggle to understand and combat these different threats. For example, in many fanfics, the characters have to figure out how to fight an alien - possessed demon or something similar.
Character consistency is a key element. The characters from Torchwood should act in a way that is true to their original personalities. For example, Captain Jack Harkness should be flirty and fearless as he is in the show.
Well, Jack Harkness from Torchwood would probably be quite intrigued by Sherlock's deductive abilities. He might initially test Sherlock, throwing out strange cases to see how he solves them. Sherlock, on the other hand, would likely be a bit put off by Jack's flamboyant nature at first but would soon realize his value in dealing with the more otherworldly aspects of cases.
One interesting plot could be where the Torchwood team investigates a series of strange murders that seem to have a supernatural element, and they enlist Sherlock's help. Sherlock, with his deductive skills, starts to piece together clues that the Torchwood team might have overlooked. For example, he could notice a pattern in the victims' backgrounds that leads to a bigger conspiracy.
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.