Well, for a One Piece Doflamingo crossover fanfiction, we could think about a crossover with a fairy - tale world. Picture Doflamingo in a world of Snow White and the Seven Dwarfs. He would be the ultimate villain, trying to take over the magic of the fairy - tale land. He could use his powers to control the dwarfs and try to capture Snow White for his own dark purposes. Another option is a crossover with a superhero world. Doflamingo could be a supervillain who has to face off against a group of superheroes, and his unique powers would make for some very interesting battles.
Another plot could revolve around an OC who is a former victim of Doflamingo's cruel rule. This OC could be on a quest for revenge. They might start by gathering allies who were also affected by Doflamingo and then plan a elaborate scheme to take him down. Along the way, they could face many challenges and obstacles set by Doflamingo and his loyal followers.
One idea could be that your pet Doflamingo has a unique ability that helps the Straw Hat crew in a strange way. For example, it can sense danger from far away, like when the crew is about to be attacked by the Marines. It might use its flamboyant appearance to distract the enemies while the crew escapes.
One character that would make for an interesting pairing with Doflamingo in a crossover fanfiction is Zoro. Zoro's strong sense of justice and his skills as a swordsman would clash with Doflamingo's devious ways. They could have epic sword battles, and Zoro might be the one to cut through Doflamingo's strings, both literally and metaphorically. Also, Nami could be paired with Doflamingo. Nami's intelligence and ability to manipulate situations for her own gain, although in a more positive way compared to Doflamingo, could lead to some really interesting interactions. She could try to outwit him in a game of wits.
One idea could be that Carrot discovers a lemon - themed island that has a strange power. Maybe the lemons on this island can grant temporary super - speed, like her Sulong form. And the One Piece crew arrives there. Another could be a fanfic where Carrot has to solve a mystery involving a lemon - scented treasure map that leads to a part of the One Piece. Or perhaps a story where a lemon - loving villain tries to capture Carrot for some devious plan related to the secrets of the One Piece.
One idea could be that Whitebeard is transported to a different time in the One Piece world. Maybe he ends up in the future and has to deal with new pirates and powers that he's never seen before. Another could be a crossover with another anime world, like he somehow gets stuck in the Naruto world and has to find his way back while dealing with ninjas.
Perhaps a villain from DBZ, like Frieza, finds a way to travel to the One Piece universe. He starts enslaving the pirates and taking over islands. Luffy and his crew would be the first to resist, but they quickly realize they need help. Goku and Vegeta then arrive and have to teach the pirates about ki and how to fight against Frieza's powerful minions. It would be a great opportunity to see the different fighting styles blend together.
One idea could be Mihawk getting transported to a different part of the One Piece world where he has to team up with the Straw Hats to face a new and powerful enemy. Maybe a group of ancient sea gods that are awakened and threatening to destroy all pirates. The Straw Hats could initially be wary of Mihawk but soon realize his skills are crucial for their survival.
One idea could be Ace crossing over into a modern - day world where he has to adapt to new technologies while still having his devil fruit powers. He might get involved in some sort of street racing using his fire powers to boost his vehicle.
One idea could be that the characters from Khr end up in the One Piece world and have to navigate the Grand Line. They might join a pirate crew and use their unique powers from Khr in battles against other pirates or the Marines. Another plot could involve a Devil Fruit user from One Piece ending up in the Khr world and causing chaos with their powers there.
A cool plot could involve the Pokemon world's Team Rocket equivalents trying to steal the One Piece. They might think it's some sort of ultimate Pokemon - related power source. Meanwhile, the Pokemon trainers in the One Piece world could form alliances with the pirates. For example, a water - type Pokemon trainer could team up with Sanji on a ship, using their Pokemon's water abilities to help with cooking and other ship - related tasks.