I'm not really into this kind of fanfiction much. But generally, in such fanfictions, the relationship between Doflamingo and Law might be explored in a very different way from the original story. Maybe it focuses on their power dynamics and how they interact with each other in a more personal or emotional level.
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.
First, consider their backstory. For example, if your OC grew up in a small island that was once under Doflamingo's control but managed to escape, it would shape their personality. They might be cautious, distrustful of authority, and have a strong will to protect others from a similar fate. Their skills could be related to survival, like being an excellent hunter or fisherman, which they learned during their escape.
If Doflamingo and the female Harry Potter character were forced to work together, it would be a very tense relationship. Doflamingo would constantly be testing her limits, while she would be on guard against his sudden betrayals. He might use his powers to intimidate her, but she would respond with her own brand of courage and magic, making their interactions full of drama and conflict.
First, give your OC a clear motivation. If they are against Doflamingo, it could be revenge for something he did to their family. If they are with him, it might be for power or protection. Second, create a unique backstory. Maybe they grew up in a war - torn area and had to fight to survive. Third, give them distinct physical and personality traits. For example, they could have a scar on their face from a past battle and be very headstrong.
In a Doflamingo x Law fanfic, Doflamingo's cruel streak can be a central trait. It can create a contrast with Law's more empathetic side. Law's loyalty to his crew could also be explored. He might have to choose between his loyalty to them and his interactions with Doflamingo. Doflamingo's self - centeredness is another trait that can be used to create conflict with Law, who often thinks about the greater good.
For Doflamingo, you can focus on his flamboyant and unpredictable nature. Show his sadistic side in how he deals with enemies, but also explore his more vulnerable moments, like when his plans are thwarted. For Crocodile, emphasize his cool and calculating demeanor. Let his past failures fuel his actions in the fanfiction, like his determination to regain his lost status. Maybe show how Doflamingo's chaos sometimes clashes with Crocodile's order.
I really like 'Doflamingo's Unseen Side'. It presents a different perspective on Doflamingo. It has a lot of action scenes and also explores his relationships with other characters in the One Piece universe in a unique way.
Luffy being raised by Doflamingo would be a radical departure from his normal upbringing. Doflamingo's way of life, which involves a lot of underhanded tactics and a disregard for the well - being of others, would shape Luffy in a very different way. He may not have the same innocent and carefree attitude. His goals could be more self - centered, like taking over territories or amassing power for the sake of it. His relationships would also be different. Instead of forming deep bonds of friendship, he might form alliances based on power and convenience. And his approach to challenges would be more in line with Doflamingo's cut - throat style, rather than his own original tenacious but more morally - guided approach.
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.