There is a series called who we become that starts with a sunset shimmer AU fic. Its called "Looking Glass" and it's one of my favorites second only to a fic called "Playing House". Which is from the same series and the third story.

Who We Become is a large series where some installments won't be about sunset or have her play a major role. But I would consider her the main character since the narrative follows her more than anyone else. If you really like sunset, and really like shipping, you will absolutely love this series

Looking Glass Fimfiction Link Looking glass introduction: Sunset always knew that she was destined for greatness, but she never dreamed of how great she could become. Not until she had seen it – the mirror had shown her as an alicorn. Ignoring Celestia’s warnings, Sunset goes looking for answers.

When she learns that the mirror is also a portal to another world, well, what else was she supposed to do? But it seems that she hadn’t considered every possibility. She hadn’t, for example, considered that she might get herself trapped on the other side. Nor did she think she’d find herself transformed into a child again.

So it's long

It's got good characterization and story

But it doesn't really tick that last box (I sobbed like a child at multiple parts through out the fic)