I personally would want it to be about a survivor of newborn army trying to adjust to life as a vampire or the end of the Volturi by having the main character be someone with a shady past that meets up with the Cullens and as this new vampire tries settling in with their family they discover he's a former member of the Volturi and a once founding member that and an old flame of Jane's.

Personally if the writer ever went with the Volturi route I think it would be interesting if just to explore their characters more. I'd also like to see if the Cullens would ever help a former member of the Volturi especially one that once held a high position and was turned by Aro.

Plus if such a character was introduced and it came out he faked his death I'd love to see how Aro would react to seeing someone he ordered killed alive and well.