One common theme could be redemption. Kovu has a complex past and his relationship with Simba can be a vehicle for exploring how he can redeem himself in Simba's eyes or in the eyes of the pride.
Another theme might be leadership. Since both Kovu and Simba are important figures in the 'The Lion King' universe, fanfics could explore how their relationship affects the leadership of the pride. Maybe they have different ideas on how to lead and they have to find a compromise through their relationship.
Family is also likely to be a theme. Simba has his own family and Kovu is from a different 'lineage' so to speak. Fanfics might delve into how their relationship impacts the family dynamics, both within their immediate families and the larger pride family. There could be stories about acceptance, rivalry within the family context, or how they come together to protect the family unit.
Look to Chapter 5 for the clearest answer. It begins with Actually, when Manager Fang saw Jiang Li, he was somewhat surprised., keeps building through Because she was really too beautiful, and very young, completely different..., and lands on Recently, he did suspect that his wife was cheating on.
Chapter 703, "Slap Oneself", is the clearest early answer because it turns Dragon Ridge Island's situation into a concrete plot event. The chapter turns that into something concrete by moving from Wu Daoshan quietly raised his head to size up Zhao Lang, and sure...
Chapter 813, "She Has a Child?!", is the clearest early answer because it turns Meng Xuezhi's situation into a concrete plot event. The chapter turns that into something concrete by moving from In the past, he might have helped Meng WanZhi speak.
Chapter 735, "How is it Him?!", is the clearest early answer because it turns Luo Xingmu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Besides..." Yin Le hesitated, "Changqing originally didn't let me say...
Chapter 895, "Love Is Not Something You Can Buy or Sell at Will", is the clearest early answer because it turns Dai Jing's situation into a concrete plot event. The chapter gives readers a real answer by opening on Dai Jing also joined in with a gossiping look: "Which young...
Chapter 734, "Are These Primary School Students Arguing?", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. It opens with Old Sir Jiang was especially happy. and closes on Jiang Yan had to return to the set as well..
Chapter 700, "As long as the money is in place, I am willing to do anything!", is the clearest early answer because it turns Qin Tianxiang's situation into a concrete plot event.
Chapter 639, "Young Master Gu, Does Your Family Need a Dog?", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. Early on, Half an hour later, Jiang Li pushed open the door and walked out..
Chapter 693, "Is a Man's Prime That Short?", is the clearest early answer because it turns Is's situation into a concrete plot event.