A great storyline might involve a fusion of the two worlds' mechanics. For example, the Velvet Room from Persona could have some connection to Joker's world. The Igor-like figure could offer Joker and his friends some new kind of power or knowledge that they use to combat a common foe. Or perhaps the Persona characters learn about Joker's Metaverse Navigator and try to replicate it in their world, which leads to all sorts of unforeseen consequences and adventures.
There could be a story where Joker's identity is somehow merged with a Persona character. He has the memories and powers of both, and has to navigate the two worlds while trying to keep his new, complex identity a secret. This would create a lot of internal conflict as he tries to balance the different aspects of his new self, all while facing external threats from those who would take advantage of his unique situation.
The allure of a female Joker in Persona 5 fanfic lies in her ability to redefine the roles within the group. She can be a symbol of change and progression. In terms of combat, she could use her femininity in a strategic way, perhaps using it to deceive enemies or gain an advantage in a different way than the male Joker. Also, her relationships with the confidants could be more complex, as she might face different reactions from them due to her gender.
Perhaps a plot where the Persona characters are invited to a special tournament in the Senran Kagura world. The tournament has strange rules that are different from what they are used to. As they participate, they start to notice some dark undercurrents. It turns out that an enemy is using the tournament to drain the energy of the combatants for a nefarious plan. The Senran Kagura and Persona characters must then put aside their differences and work together. The Persona characters can use their Personas to analyze the situation and the Senran Kagura characters can use their combat expertise to fight the physical battles.
Another could be a time - travel scenario where Naruto goes back in time and meets Rikudou Sennin. They then team up to prevent a great evil from ever arising in the first place. This would allow for a lot of character development and exploration of the Naruto universe's history.
One of the most interesting storylines could be the interaction between the different species in Mass Effect and those from Stargate. For example, how would the Asari react to the Goa'uld? The Asari are a long - lived, biotic - using species in Mass Effect, while the Goa'uld are parasitic snake - like beings in Stargate. Another cool idea is the technology exchange. The Mass Effect universe has advanced biotic and cybernetic technology, and Stargate has its own unique wormhole - based travel and Ancient technology. A fanfiction could explore how these technologies might be combined or used against each other. Also, the cultural clashes would be fascinating. The different races in Mass Effect have their own moral and ethical codes, and the same goes for the races in Stargate. How would they deal with each other's values?
One interesting storyline could be Naruto using the Sharingan to enhance his combat skills in a new and unexpected way. For example, he might combine his Rasengan with Sharingan - powered genjutsu to create a more powerful and deceptive attack.
A crossover involving a shared enemy. Maybe a new alien race that is a threat to both universes emerges. The combined forces of Babylon 5 and Star Trek would have to pool their resources. The Starfleet officers could share their scientific knowledge to analyze the new threat, while the Babylon 5 personnel could offer their street - smart combat and negotiation skills developed from their own experiences in the often - tumultuous Babylon 5 station.
A really cool storyline might involve a powerful object or spell that links the two shows. For instance, a magical artifact in 'Supernatural' that can also affect the Wesen in 'Grimm'. The characters from both shows would have to race against time to find and control this object before it falls into the wrong hands. It would be a high - stakes adventure that combines the best elements of both shows, like the family - centered themes in 'Supernatural' and the unique fairy - tale inspired world of 'Grimm'.
Well, perhaps a story where the two characters cross paths due to a serial killer case. The killer seems to be targeting people involved in the legal system, and Perry Mason is representing some of the potential victims or suspects. Ironside gets involved as the detective on the case. Their different approaches at first clash but then they start to respect each other and combine their efforts to catch the killer. They might also discover a hidden conspiracy behind the killings.
A crossover fanfiction could also explore a personal connection. What if a member of the Reagan family was a big fan of Castle's books? And then they end up meeting in a situation where they need to work together on a case. This would add an interesting layer of fan - meets - idol dynamic to the story while also dealing with the crime - solving aspect.
One of the most interesting storylines could be about a character from Skyrim finding themselves in the world of Dragon Age. Maybe a Dragonborn suddenly appears in Ferelden during the Blight. They could use their Thu'um to fight darkspawn in a way that astonishes the people there. Another great idea is a mage from Dragon Age being teleported to Skyrim and having to deal with the different magic system and dragons there.