One idea could be an OC who is a spy sent to infiltrate Doflamingo's organization. This OC could have a hidden past that slowly gets revealed as they gain Doflamingo's trust. Another idea is an OC with a special power that Doflamingo wants to control or use for his own evil plans. For example, the OC might be able to manipulate time in a small area, which could be a great asset in battles or heists.
A good concept for a Doflamingo OC fanfiction is an OC who is a former slave of Doflamingo's but manages to escape. This OC then spends their time trying to free the other slaves and bring down Doflamingo. They could form an alliance with the Straw Hat Pirates or other like - minded groups. The OC could also have a unique fighting style developed from their years of oppression, like using chains in creative ways.
You could write about an OC who is a noble from a different kingdom. This OC comes to Dressrosa, Doflamingo's territory, for diplomatic reasons. However, they soon discover Doflamingo's true nature. The OC has to navigate the political intrigue while also trying to avoid being caught by Doflamingo. They might have a personal code of honor that clashes with Doflamingo's immoral ways.
Chapter 431, "Su Yinwan Is Here Too?!", is the clearest early answer because it turns Is's situation into a concrete plot event. The chapter turns that into something concrete by moving from [Holy shit, why is Su Yinwan on this show too???] [!!! to Is the program team this cruel?.
Chapter 685, "Defense Value Skyrockets!", is the clearest early answer because it turns Shao Changqing's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The scene went live on Weibo right then.
Chapter 12 is the strongest chapter-level answer because it starts from Qin?" Jiang Li called out twice, and only then did Mrs. and follows through on The glass shattered on the floor with a crash, attracting the attention of....
Chapter 492, "Rewarding You with Some Wild Vegetables", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 470, "Why Talk About Pigs When Looking at Him?", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. It opens with Wrapped in a bit of warmth, Jiang Yan slowly came to.
Chapter 633, "It's Time for Us to Attack!", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 736, "When Will You Come to My House for Dinner?", is the clearest early answer because it turns Such's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Walking outside the tent, Zhao Lang waved his hand.
Chapter 742, "Brother Policeman, Look at Them!", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Fang Kai became panicked as things were not developing in his favor.
Chapter 713, "Uncle Will Owe You for Life", is the clearest early answer because it turns Lili's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Li raised her head and was startled by Zhao Lang's smiling eyes.
Chapter 686, "Why Not Make Him Mute?", is the clearest early answer because it turns Luo Xingmu's situation into a concrete plot event. The chapter turns that into something concrete by moving from In the makeup room, Jiang Yan sneezed three times in a row.