Another option is to check out some RWBY - related forums or communities. Sometimes fans will share their own fanfiction or recommend their favorite stories. They might even discuss the potential of the Cinder x Mercury pairing, which could give you some ideas on what to look for in a fanfic.
One great place to start looking for RWBY Cinder x Mercury fanfiction is Archive of Our Own (AO3). There are numerous stories there that explore their relationship in different ways. Some might focus on their shared goals within the context of the RWBY universe, while others might take a more character - study approach, delving into their individual personalities and how they interact with each other.
You could also try Fanfiction.net. There are some interesting Cinder x Mercury fanfics there as well. However, the quality can vary widely. Look for stories that have a decent number of reviews and ratings. A good fanfic might explore how their relationship could develop over time, perhaps in an alternate universe where the events of RWBY take a different turn.
Chapter 653, "Everyone's Champion", is the clearest early answer because it turns Yuanyuan's situation into a concrete plot event. The chapter turns that into something concrete by moving from As dawn broke, "Wilderness Survival 30 Days" released a notice on...
Chapter 636, "Beg Me", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. It opens with Uh..." Zhao Lang scratched his nose, feeling a bit embarrassed, "I... and closes on His hair was a mess sticking to his face, his beard had grown out in....
Chapter 696, "I Am a Very Narrow-Minded Person", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 841, "Isn't this Daniu?", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. It opens with Zhao Lang took the photograph and glanced at it, chuckling, "It... and closes on Jiang Li looked at the old woman and found her quite familiar..
If you want the clearest chapter reference, go to Chapter 19. Early on, What criminal law is this or that, this was originally none of your...; later, the chapter leans on In high school, she got into the international class by throwing money....
A practical answer is Chapter 18, because it turns the idea into a concrete scene. The chapter opens with In the corner, a girl in a black dress sat quietly, holding a glass of..., adds pressure through I have no reason to joke with the rest of my life." "Moreover, I wouldn't..., and ends on Hurry.
The best chapter pointer here is Chapter 13, "Killing the Chicken to Scare the Monkey". It opens with Sensing that something was off with Jiang Yan, Jiang Yan dismissed the... and closes on How could this happen..." Lin Manru didn't want to hear any more of her....
If you want the clearest chapter reference, go to Chapter 9. Early on, Exactly, the Qiao Family isn't that great either.; later, the chapter leans on They've been yelling and shouting since morning, with no sense of decorum....
Chapter 739, "Not Paying Debts, Shameless!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter gives readers a real answer by opening on Shu Yan isn't that ill, it's just a common cold, no, no need to go to...
Chapter 653, "Everyone's Champion", is the clearest early answer because it turns Everyone's situation into a concrete plot event. The chapter gives readers a real answer by opening on As dawn broke, "Wilderness Survival 30 Days" released a notice on...