Like I said, it's kind of a boring story. There are so many better villains, and it feels wrong to not use the classics like Riddler, Penguin, and Catwoman, in favor of a character that never even appeared in the comics.
With someone like Riddler or Catwoman, you can take their decades of stories, and combine elements of all of it to come up with something original, yet true to the character. But with Phantasm, you only have one story to work with, so by including the character you're basically just retelling that one story. Last thing I want BB3 to be is a live action version of MotP.