If a prequel were to be made, I'd definately read it. Jasper's years in the army(Confederate and newborn" would be an excellent story, although maybe considered too bloody for YA...not that Tamora Pierce has any qualms writing gore.
Also, Alice as she's looking for Jasper and the other Cullens.
Or Esme during those ten years between Carlisle sightings.
There is so much that can be done.
Maybe a Quilheute story, from Jacob's grandfather's pov when they made the deal with the Cullens in the first place.
Tanya and her sisters' years as Succubi.
Anyone with any kind of skill at writing fanfiction, feel free to pick up any of my suggestions