I've actually seen a surprising amount of 'Magnificent Seven' fanfic that goes way deeper than the original series. The show gave us broad strokes with characters like Ezra, but a fic I read recently imagined his con artist days in St. Louis before he met Chris. It wasn't just about the tricks; it was about the loneliness of that life, the constant calculation, creating a whole persona to survive. That kind of backstory work makes his choice to stay with the group later feel so much weightier.
Another angle I love is exploring the 'what if' of Buck's past with the Cheyenne. The show hints at it, but fics can spend chapters on his internal conflict, the cultural dislocation, the specific moments that forged his loyalty. It adds layers to his easygoing nature that the show's runtime couldn't fully develop. Sometimes the best backstories aren't about grand adventures, but about the quiet, formative hurts that shape why a man picks up a gun, or puts one down.