Well, one common theme could be self - discovery. Both characters might learn new things about themselves through their relationship. There could also be a theme of power dynamics, especially if their abilities or statuses in their respective worlds are different. Additionally, themes of loyalty and betrayal might come into play, as their relationship might be tested in different situations.
If such a fanfic exists, it could potentially have themes of friendship turned into something more, as Killua and Gin might start from a basic acquaintance or even rivalry and develop deeper feelings. Another theme could be adventure, as they might go on some sort of journey together that changes their relationship.
Chapter 700, "As long as the money is in place, I am willing to do anything!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.
Chapter 341, "Whose Child Is Jiang Cheng Really?", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with Wei Dong glanced at his wife who had followed closely behind and...
Chapter 675, "The Hope of the Jiang Family", is the clearest early answer because it turns Grandpa's situation into a concrete plot event.
Chapter 721, "Apology", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 455, "She is Entirely My Weakness", is the clearest early answer because it turns Dean Yu's situation into a concrete plot event. The chapter gives readers a real answer by opening on The marathon-style competition ultimately ended with Shang Shuyue...
Chapter 623, "Turned into a Little Crybaby", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Jiang Cheng woke up again, this time realizing he was being carried....
Chapter 847, "How Else Can You Argue?!", is the clearest early answer because it turns Rong Kexi's situation into a concrete plot event. It opens with The Rong family couple watched the situation unfold, exchanged a...
Chapter 487, "Found a Kindred Spirit?", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event.
Chapter 735, "How is it Him?!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with Besides..." Yin Le hesitated, "Changqing originally didn't let me say...
Chapter 618, "Abandoned Again", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.