A possible plot could be that Jack the Ripper's true identity in the fanfiction is actually a corrupted form of an otherwise noble Servant. Maybe he was a heroic figure in the past but was corrupted by a powerful dark magic. A group of Fate characters could then be on a quest to find a way to reverse this corruption. They would have to face not only Jack the Ripper's deadly attacks but also the political and social unrest in the London setting of the story. The fanfiction could also explore the psychological aspect of how the Ripper's mind works and how the Fate characters react to his extreme violence.
Well, it could be that Jack the Ripper has some sort of connection to the Holy Grail in the Fate world. Maybe his killings are part of a ritual that, if completed, would grant him ultimate power. A young and inexperienced Fate Master might accidentally stumble upon this plot and has to team up with other more experienced Servants to stop Jack. Along the way, they discover that the Ripper has allies in the form of dark mages who are using him for their own gain. This would create a lot of action - packed scenes as the good guys try to outwit and overpower their enemies.
Fate characters with a more empathetic side, like Jeanne d'Arc, might try to understand what drove Jack the Ripper to become a killer. She might hope to redeem him rather than simply defeat him. Meanwhile, the more battle - hungry characters like Lancelot would be eager to test their skills against Jack. Lancelot would be drawn to the danger that Jack represents and see it as a chance to prove his worth in combat. The different reactions would create a rich tapestry of interactions within the fanfiction.
One idea could be a crossover with the Justice League. Dr Fate could use his magic to help the League against a powerful magical threat that they can't handle alone. Maybe a dark sorcerer from another dimension is trying to invade Earth, and Dr Fate's knowledge of ancient magic is crucial in devising a counter - attack.
One idea could be that the Servants from Fate are transported to the Halo universe. For example, Saber could find herself in a battle against the Covenant. Her noble phantasm could be a powerful weapon against the alien foes. Another plot could involve Master - Spartan team - pairings. A Spartan could be paired with a mage to fight a common enemy that threatens both the Halo and Fate worlds.
One idea could be that Naruto is summoned into the Fate universe during the Holy Grail War. He might be mistaken for a Servant at first. His unique ninja abilities would disrupt the normal flow of the war. For example, his Shadow Clone Jutsu could be used to confuse the enemy Masters and Servants.
A good plot for Eyeless Jack crossover fanfiction might be a crossover with a magical realm. Imagine Jack finding himself in a world of wizards and witches. He would be an enigma to them, and his appearance and abilities would likely cause a stir. They might try to either banish him or use him for their own purposes. Another option is a crossover with a horror - comedy universe like "Ash vs Evil Dead". Jack's serious and menacing nature could create some really funny and tense situations when contrasted with the more over - the - top and comical elements of that universe. Also, a crossover with a time - travel story could be interesting. Jack could be sent to different time periods and have to adapt to the different cultures and threats in each era.
For 'fate crossover fanfiction', you could consider a crossover with 'One Piece'. The Servants might be pirates or marines, sailing the Grand Line. A 'Fate' and 'Fullmetal Alchemist' crossover would also be cool. Picture Servants trying to solve the mysteries of alchemy. There's also potential in a 'Fate' and 'My Hero Academia' crossover, with Servants having Quirks.
Maybe a time - travel based crossover. A character from Fate Kaleid Liner accidentally travels back in time and meets the younger versions of other characters in the series. This could lead to changes in the timeline and they would have to work together to fix any potential disasters that their meeting has caused, all while trying to find a way back to their own time.
Maybe Arthur Pendragon gets transported to the Fate world and his presence disrupts the normal balance of power. The mages in the Fate universe might see him as a threat or an opportunity. He could end up being a wild card in the Grail War, not bound by the normal rules of Servant summoning. His noble phantasm, Excalibur, could be a game - changer in battles against other powerful Servants.
A possible plot for this crossover fanfiction could be centered around the concept of the 'ideal'. In Fate Stay, each Servant has an ideal they fight for, and in RWBY, the characters also have their own beliefs and goals. So, a story could be crafted where the characters from both universes have to compare and contrast their ideals. Maybe some of the Fate Stay characters are confused by the concept of the Grimm and the Huntsmen/Huntresses' role in RWBY, while the RWBY characters are intrigued by the Servant - Master relationship in Fate Stay.
Maybe a character from Blue Exorcist gets summoned into the Fate universe by mistake. They then have to adapt to the new rules and magic there, and form alliances with the Fate characters to find a way back home. Along the way, they discover new aspects of their own powers.