Now, when the doors of death are open I'm sure Gaea can't control all souls that escape from the underworld. Bianca di Angelo might be also able to come back to life. She could be the child of Hades/Pluto in the second Great Prophecy. Remember, in The Titan's Curse, Zoe said Bianca was a very powerful demigod, Zoe even believed Bianca showed the potential to replace her. Bianca would also represent the Hunters in the prophecy.
What do you think about the idea of Bianca being one of the 7? If you don't like it, who do you think is the child of Hades/Pluto?