The reason the Cullens actually left in New Moon was due to a threat in the Vampire World, which causes vampires to become human, but in the state they were just before they were turned (e.g. Edward is a human about to die from the Spanish Influenza, but Carlisle is an otherwise healthy human).

It is up to you whether or not Bella was informed of this threat.

The only survivors of this threat (out of the Cullen Coven) are Carlisle, Jasper and Alice.

It is up to you how widespread this vampires-becoming-human threat is (who else survived / died / was affected)

It is up to you as to the nature of the threat (a vampire-affecting disease, a particular vampire's gift etc).

Alice's mental state is questionable (can her human brain handle her gift?).

It is up to you how to handle Jasper's gift in his human state.

Bella assists the vampires to acclimate to being human again.

Preferred pairing either Bella/Carlisle or Bella/Jasper.

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This kind of plot has been swimming around my brain for a long time - I would love to see it created into a story but can't do it myself because I can't write to save my life. Hopefully somebody out there will pick this up and be able to run with it.