Feyre's evolution in fanfic is almost entirely about unpacking the aftermath of the 'A Court of Mist and Fury' trauma and subsequent power-up. In canon, she moves into a more assured role as High Lady pretty fast, but fanfiction likes to slam the brakes on that. A huge chunk of stories explores what happens when the confidence is a front, when the lingering panic from Under the Mountain resurfaces at the worst moments. I've read so many one-shots where a casual touch from a stranger sends her spiraling, or a nightmare has her reaching for Rhys only to find him already awake and having sensed it through the bond. It's that daily, domestic management of PTSD that canon glances over but fic revels in.
Another major thread is the political side of her evolution, but often with a critical edge. Fics will position Feyre as someone deeply uncomfortable with the pageantry of ruling, making mistakes in diplomacy because her instinct is still to act as a huntress—direct and forceful. There's a popular AU where she never became High Lady and instead works as a mediator between the courts, using her understanding of the human world and the faerie realms to bridge gaps, her evolution measured in treaties brokered rather than battles won. It's a slower, more intellectual growth, and it often feels more earned than the sudden authority she gets in the books.
Finally, there's the 'what if' divergence evolution. What if she hadn't been resurrected by the High Lords? What if the mating bond with Rhys never snapped into place? I've seen darkfics where she becomes a bitter, mercenary figure operating in the shadows of Velaris, or melancholic pieces where she remains Tamlin's wife and slowly, quietly, teaches herself to read and write as an act of quiet rebellion, her evolution confined to the private pages of a journal. That kind of character study, where her core stubbornness adapts to a different cage, is where fanfiction really redefines her arc.