That's a fun question because Narnia feels particularly suited for original character stories, given how big and ancient that world is. For me, the best ones don't just plop a new person into the plot of the books, but find a corner of the world that C.S. Lewis only hinted at and build something there.
I kept thinking about 'The Last King' by Athelas83, which follows an original Calormene noble in the generation after 'The Horse and His Boy'. It's fascinating because it works with the cultural world-building, which in the books is pretty thin, and makes it complex. The OC isn't a secret hero who fixes everything; she's navigating politics and faith, and her journey feels earned. The writing is very atmospheric, like you can feel the desert heat.
Another standout is 'A Stone Gryphon' on Archive of Our Own, a multi-generational story about a Telmarine family from before the Pevensies' return all the way through to Caspian's reign. The OCs are witnesses to history rather than driving it, which gives it a quiet, almost historical fiction vibe that really works for Narnia. You see the fear and superstition during the White Witch's rule, and the slow, messy change when the kings and queens come back. It's less about epic battles and more about how a world heals, which I found really moving.
Honestly, a lot of the ones set during the Golden Age can feel samey, but these two stood out because they chose different eras and perspectives. They treat Narnia as a living place with its own problems, not just a backdrop for the familiar characters.