Howling:Cancelled novel

The blurb of the 2005 cancelled novel "Rain of Terror" is:

Plot Outline: The Year is 2050. The Earth has been hit by a monumental flood, the worst seen since the dawn of time. But The Doctor, Rose & Captain Jack discover that something other than Nature itself could have caused this terrible downpour... The Doctor is about to face one of his most mortal enemies, who he had believed was long since dead...
 * Does anyone know/think they know who or what the "mortal enemy" would have been if this book had been published?

Plot Outline: The Year is 2050. The Earth has been hit by a monumental flood, the worst seen since the dawn of time. But The Doctor, Rose & Captain Jack discover that something other than Nature itself could have caused this terrible downpour... The Doctor is about to face one of his most mortal enemies, who he had believed was long since dead...
 * Does anyone know/think they know who or what the "mortal enemy" would have been if this book had been published?