One interesting plot idea could be a character from the Bloodborne world being transported to the realm of the Old Ones. They could struggle to adapt to the strange and eldritch environment while also trying to find a way back home. Maybe they discover secrets about the Old Ones that could change the very fabric of both worlds.
One interesting plot idea could be that Fem Naruto, with her ninja skills, enters the Twilight world. She might get involved in the vampire - werewolf conflict. For example, her ability to sense danger could be a great asset. Another idea is a love triangle between Fem Naruto, Edward, and Jacob. Her strong - willed nature would add a new dynamic to the relationship.
One interesting plot idea could be a battle between the hunters from Supernatural and the vampires from Vampire Diaries. Maybe the Winchesters end up in Mystic Falls and think all the vampires there need to be hunted. Another idea is a character swap. For example, Castiel could end up in the Vampire Diaries world and interact with the characters there in a very different way compared to his usual interactions in Supernatural. A third idea could be a joint mission to fight a greater evil that threatens both universes, like an ancient demon that wants to use the power of the vampires in Vampire Diaries and the magic from Supernatural for its own dark purposes.
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.
One interesting plot idea could be the combination of the technology from Mass Effect and the world - building of Anthem. For example, Shepard could find himself in the world of Anthem and have to adapt to the javelin exosuits while also dealing with the local threats. Maybe the Reapers start to target the world of Anthem as well, and the two groups from different universes have to team up to fight this common enemy.
One idea could be that the Grimm in RWBY are somehow related to the beasts in Bloodborne. Maybe there's a common origin or a dark force that controls both. For example, the Grimm could be an earlier form of the Bloodborne beasts, and the Huntsmen and Huntresses have to deal with a new type of Grimm that has the characteristics of the Bloodborne creatures, like a more sentient and intelligent type of Grimm.
A way to explore madness in the crossover is by having a character from the Bloodborne world try to use the knowledge or power of the Old Ones. Since the Old Ones are so far beyond human understanding, this attempt could backfire horribly. The character might start to lose their grip on their identity, their memories becoming muddled with the strange knowledge of the Old Ones. This transformation into madness could be a central part of the fanfic's plot, as the other characters then have to deal with this once - sane individual who has now become a threat due to their new - found madness.
One interesting plot idea could be Naruto's chakra system interacting with magic in the Harry Potter world. For example, his ability to use shadow clones might be seen as a form of magic by the wizards. Another idea could be Naruto being sorted into a Hogwarts house, perhaps Gryffindor because of his courage. Or maybe he could teach the Hogwarts students some of his ninja skills in a special class.
One idea could be Master Chief being thrown into the Mass Effect universe during a crucial moment in the fight against the Reapers. His combat skills and Spartan abilities would surely turn the tide in unexpected ways. For example, his mastery of close - quarters combat could be a game - changer in battles where the Mass Effect characters are more used to ranged combat with biotics and guns. Another plot idea could be about the technology exchange. The Covenant technology that Master Chief is familiar with and the advanced tech in the Mass Effect universe might create some really interesting scenarios. Maybe the UNSC and the Citadel Council could form an alliance based on shared technology interests.
One interesting plot idea could be the Spartans from Halo Wars 2 teaming up with the Jedi from Star Wars. Imagine the Spartans' combat skills combined with the Jedi's use of the Force. They could fight against a common enemy, like a powerful Sith Lord who has somehow got hold of Covenant technology.
Well, a cool concept is the power dynamics. In fanfiction, the Old Ones are extremely powerful entities. So, in a crossover, they could disrupt the power balance in Bloodborne. They might introduce new forms of blood magic or madness that the Hunters have never seen before. Also, the cultural clash between the two universes can be fascinating. The Old One's way of existence might be completely different from what's known in Bloodborne, leading to all sorts of interesting plot developments.