How about a crossover with a post - apocalyptic world? Kid Gilgamesh could bring his ancient powers and knowledge to a world that has been ravaged. He might have to decide whether to rule over the survivors or help them rebuild, all while dealing with the new threats that exist in this desolate world.
One plot idea could be Kid Gilgamesh crossing over into a modern - day world and having to adapt to modern technology while still maintaining his pride and power. Maybe he gets into battles with modern superheroes who underestimate him at first.
Maybe Gilgamesh gets involved in the power struggles within the DxD universe. He might see the different factions as beneath him but still decides to meddle for his own amusement. He could try to reshape the power structure according to his own ideals, which would lead to many interesting confrontations.
A plot for Gilgamesh Danmachi fanfiction might be that Gilgamesh discovers a hidden part of the dungeon that has a connection to his past. Maybe it contains ancient artifacts or secrets related to his origin. As he delves deeper, he has to team up with some of the local adventurers from Danmachi. They are initially skeptical of him but soon realize his power and knowledge are invaluable. Together, they face various challenges and traps, and Gilgamesh also has to deal with his own internal struggles as memories from his past start to resurface.
Another interesting concept is a crossover with the Harry Potter universe. Picture Kid at Hogwarts. He'd probably be a bit of a troublemaker, not fitting in well with the school rules. He could form an unlikely friendship with the Weasley twins and together they'd play pranks on the Slytherins, all while Kid is trying to find a way back to his own world.
A cool Kaito Kid crossover fanfiction idea is with the world of Harry Potter. Kaito Kid could try to steal a magical artifact from Hogwarts, like the Philosopher's Stone. He'd have to navigate through the magical defenses and deal with the wizards and witches. Also, a crossover with One Piece. He might target a Devil Fruit, which would put him in conflict with the Straw Hat Pirates. And a crossover with Pokémon. He could be after a rare Pokémon that has some special power, like Mewtwo.
One idea could be a crossover with Harry Potter. Kid Goku could end up at Hogwarts and his martial arts skills would be a unique addition to the magical world. He might use his Ki blasts instead of magic spells in some situations.
One idea could be a crossover with the Teen Titans. Kid Flash could team up with them to fight a new super - villain that is causing chaos across multiple cities. His speed could add a new dynamic to the Titans' combat strategies.
One idea could be a crossover with the Dragon Ball Super universe. Kid Buu could encounter new and powerful foes like Jiren. Their fight would be chaotic and destructive, with Kid Buu's unpredictable nature causing all sorts of mayhem. Another idea is a crossover with the Naruto universe. Picture Kid Buu facing off against Naruto. Naruto's positive energy and willpower could be a contrast to Kid Buu's pure evil. He might try to reason with Kid Buu at first, but when that fails, a huge battle would ensue. And a third idea could be a crossover with the One Piece world. Kid Buu might land on the Grand Line and start causing trouble for the Straw Hat Pirates. Luffy would surely step up to protect his crew and the islands they visit from Kid Buu's rampage.
One idea could be that Gilgamesh, with his vast power and arrogance, arrives in the RWBY world. He might initially see the Huntsmen and Huntresses as beneath him but then becomes intrigued by Ruby's unique silver - eyed power. Another could be a scenario where Gilgamesh allies with a villain in RWBY, like Cinder, and together they try to reshape Remnant according to their desires.
One idea could be that Percy and his friends encounter Gilgamesh during a quest in a strange new realm. Gilgamesh could be a powerful ally or a mysterious foe. Maybe they have to work together to defeat a common enemy that is threatening both the world of Percy Jackson and the ancient lands of Gilgamesh.
One idea could be the characters from Kid Icarus and High School DxD teaming up to fight a common supernatural threat that combines the mythologies of both universes. For example, a demon - angel hybrid that also has ties to the Greek mythology in Kid Icarus.