One idea could be to have characters from different universes within the 'devil survivor 2' world meet and form unexpected alliances. For example, a group of normally opposing demon summoners having to work together to face a greater threat that endangers all of their worlds.
A really cool plot could be a crossover where the characters from 'devil survivor 2' end up in a parallel universe that has a different set of rules regarding demons and survival. They would have to adapt quickly and figure out how to use their skills in this new and strange environment. This could lead to a lot of character development as they struggle to understand and overcome the challenges.
One interesting plot idea could be a crossover between the Devil May Cry universe and the Castlevania universe. The protagonist with Devil Trigger ability could find himself in the dark and haunted world of Castlevania. He has to use his powers to fight off the hordes of demons and undead creatures that are native to Castlevania. Maybe he teams up with a Belmont character and they exchange combat techniques and knowledge about their respective worlds.
One idea could be a survivor in a post - apocalyptic world who has to deal with not only the usual threats like lack of food and water but also new, mutated creatures. Another could be a group of survivors on a deserted island where they discover a hidden, ancient civilization. Or perhaps a survivor in a war - torn city who forms an unlikely alliance with an enemy combatant to stay alive.
Another interesting concept is to cross the devil fruits with the world of Harry Potter. Imagine a character with a devil fruit ability like the ability to turn into an animal (zoan type) trying to blend in at Hogwarts. They might have to hide their powers from the wizards while also using them to their advantage in the magical world, like using their animal form to explore hidden areas of the school.
A possible plot is a magical artifact from Fairy Tail's world that is sought after by demons in Devil May Cry's universe. The Fairy Tail gang teams up with the Devil May Cry crew to protect it. The artifact might have the power to either destroy all magic or enhance it to uncontrollable levels. Dante and Natsu could have a friendly rivalry over who is stronger and who gets to fight the main villain. Another plot idea is that some of the Devil May Cry characters are sent back in time to the Fairy Tail world's past. They have to adapt to the magic - based society and help the Fairy Tail founders with a great threat that was once thought to be long forgotten.
One idea could be a crossover with 'The Texas Chainsaw Massacre' family. The Devil's Rejects could team up with the Sawyers in some sort of mayhem - filled adventure against a group of bounty hunters. They could combine their unique styles of horror and chaos.
One idea could be the lone survivor Naruto having to navigate a world where all his friends have been put under a powerful genjutsu. He has to find a way to break the spell alone, facing many hidden enemies along the way.
One idea could be Percy using his devilish powers to protect his friends in a unique way. Maybe he has the power to manipulate shadows and uses this to create illusions to mislead enemies.
For 'a devil is unbreakable fanfiction', you could have a plot where the devil is forced to take on a human form for a period of time. In this form, it experiences human emotions like love and compassion for the first time. This could create a conflict within the devil as it tries to reconcile these new feelings with its devilish nature. Additionally, a plot about the devil being hunted by a group of powerful sorcerers who want to use its unbreakable essence for their own dark purposes would be exciting.
Pairing a tech - savvy character from 'devil survivor 2' with a magic - using character from another world could be really interesting. The tech character could be fascinated by the magic, and the magic user could be intrigued by the advanced technology in the 'devil survivor 2' world. They could combine their skills in unexpected ways to face challenges, like using technology to enhance the magic or using magic to power up technological devices.
One idea could be a crossover with the Bayonetta series. Dante and Bayonetta could team up to fight a common, powerful demon. Their different fighting styles would make for some epic battles and interesting interactions. For example, Dante's flashy swordplay combined with Bayonetta's acrobatic and magical combat.