Well, perhaps there could be a plot where an old enemy of the Doctor's resurfaces and targets Torchwood to get to the Doctor. This would force the two groups to work together. The Doctor could bring his knowledge of the universe and different species, while Torchwood could contribute their understanding of Earth - specific threats and their more hands - on, military - like approach. It could also be interesting if there was a fanfiction where a member of Torchwood gets accidentally sent to a far - off planet and the Doctor has to rescue them and in the process, they discover a new form of energy that could change the future of both their worlds.
One idea could be that they team up to face a new alien threat that is beyond what either of them has faced alone. Maybe an alien that can manipulate time in ways that disrupt the fabric of reality. Another could be a story where the Doctor and the Torchwood team have a clash of ideologies at first regarding how to handle a situation with a lost alien civilization on Earth, but then they learn from each other.
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.
Well, in this fanfiction, there could be a lot of exciting crossovers. Maybe the Torchwood team and the 11th Doctor team up to fight some new and powerful alien threats. They might share their different technologies and knowledge. For example, the Doctor's TARDIS could be used in a new way with Torchwood's high - tech equipment.
Well, there could be all sorts of adventures. Maybe they'd team up to fight a new alien threat that neither could handle alone. The Doctor's wacky and brilliant ideas combined with Torchwood's more grounded, Earth - based tactics could make for some really interesting scenarios.
A storyline could involve a character from Torchwood getting trapped in a time loop. The 11th Doctor shows up and, with his knowledge of time mechanics, tries to break the loop. Along the way, he gets to know the Torchwood team better and they learn a great deal from him about the nature of time and the wider universe.
There might be a fanfiction where Rose joins Torchwood on a mission to a parallel universe. In this story, she could bring her perspective from different dimensions and time travels, and together with Torchwood, they have to deal with the differences and similarities between the two universes while facing some common threats.
One idea could be a fanfic where Torchwood has to deal with an alien invasion that targets Earth's power sources. They'd have to use their unique skills and technology to stop it.
The main characters would surely include the 11th Doctor. He would bring his TARDIS, sonic screwdriver and all his time - lord knowledge into the mix. From Torchwood, Toshiko Sato could be a significant character. Her expertise in technology and her rather shy yet intelligent personality could add depth to the story. She could potentially help the Doctor with some of his more complex technological problems, and in return, the Doctor could expose her to things she had only dreamed of in her scientific pursuits.
Well, of course, the Doctor is always a popular focus. But from Torchwood, Toshiko Sato is quite interesting. Her technological expertise could be very useful when combined with the Doctor's knowledge. She could also learn a great deal from him. Another character that can't be ignored is Martha Jones. Although she has her own story with the Doctor, when she gets involved with Torchwood in the fanfiction, her medical skills and her past experiences with the Doctor can create a whole new set of storylines and character relationships.
It could bring together beloved characters from both shows in unexpected ways. Maybe there's a shared adventure that combines their unique skills and storylines.
Perhaps a fanfiction where Doctor Strange loses his powers and has to rely on his wits and the knowledge of ancient spells he has memorized to defeat a powerful enemy. This would show his growth not just as a sorcerer but as a person. For example, he could use his medical knowledge in combination with some basic magical principles to outsmart the foe.