The 'redemption' trope works well for both. In Grey's Anatomy, a doctor who has made a big mistake tries to make up for it. In X - Men, a former villain mutant tries to redeem themselves. A fanfiction could be about a character from Grey's Anatomy who has a dark past (like accidentally causing a patient's death) and then discovers they are a mutant and uses their new powers to try and redeem themselves in the eyes of both the medical community and the mutant community.
In Grey's Anatomy fanfiction, the 'forbidden love' trope is quite popular. This could be between two doctors who are in different positions in the hospital hierarchy or have some other obstacle to their relationship. In X - Men fanfiction, the 'outsider' trope is common. New mutants coming into the X - Men group and feeling like they don't belong at first. When combined, you might have a story where a Grey's Anatomy character who is an outsider in the hospital (maybe a new intern) discovers they are a mutant and feels like an outsider in both worlds.
For Grey's Anatomy, the 'medical mystery' trope is a staple. In X - Men, the 'discovery of new powers' trope is popular. Fanfiction could merge these by having a character in Grey's Anatomy suddenly start exhibiting strange symptoms that turn out to be the emergence of mutant powers. The doctors then have to figure out not only how to stop any negative effects but also understand what these new powers are and how they can be controlled.
If you want the clearest chapter reference, go to Chapter 35. Early on, Shining Transformation Trip's filming location was set on the distant...; later, the chapter leans on In order to help the guest stars get better acquainted with one another,....
Chapter 843, "Why the Sudden Change of Heart?!", is the clearest early answer because it turns Gu Qiang's situation into a concrete plot event. The chapter turns that into something concrete by moving from Uncle Zhao also expressed that they did all this for money. to Gu Yu is my real grandson;
The best chapter pointer here is Chapter 27, "Signing Jiang Yan is fine, but you have to follow along too.". It opens with They had spent an entire year gradually inviting a few people. and closes on Yes, I'm sorry." He nervously twisted his hands, "I was impulsive." "It's....
The best chapter pointer here is Chapter 46, "Why Am I Second to Last?!". It opens with Upon hearing this, everyone but Jiang Li perked up with high alert. and closes on Did you calculate the score incorrectly?!" And 13 points?!.
If you want the clearest chapter reference, go to Chapter 3. Early on, Qiao was still annoyed at the attitude of this unknown girl when she heard...; later, the chapter leans on She's actually Miss Jiang Li from the Jiang Family?!.
Look to Chapter 69 for the clearest answer. It begins with Since the phones issued by the program team couldn't log into Weibo, Wen..., keeps building through However, He Yun, who had witnessed the incident, knew a bit about it., and lands on Wen Jiaojiao immediately hugged her arms proudly.
Chapter 941, "In Which Year Did the River Moon First Shine on People [Extra · River Moon Edition]", is the clearest early answer because it turns Shang University Scholar's situation into a concrete plot event.
Chapter 22 is the strongest chapter-level answer because it starts from Hearing this, Su Yinwan looked at Shang Shaojing with a face full of... and follows through on Jiang Li's righteousness temporarily shut Jiang Yan up, but after a few....
If you want the clearest chapter reference, go to Chapter 51. Early on, Oh, okay." After uttering two syllables indifferently, Jiang Li bent down...; later, the chapter leans on By the time she reacted, Jiang Li had already disappeared at the end of....
Look to Chapter 45 for the clearest answer. It begins with Helplessly, Jiang Yan had no choice but to step on the accelerator., keeps building through As Jiang Li had said, this stretch of road was indeed difficult to..., and lands on The guests have to go through so many troubles on the way.