The first movie was pretty forgettable, and I don't really see the point of a sequel. But the first movie made over $293 million. Johnston is the second writer to take a crack at the script for the sequel. Last year the screenwriter of Salt, Kurt Wimmer, was hired to write it but Jolie didn't care for the script he wrote -- which is kind of funny because the script for the first movie wasn't all that great either.
Maybe Johnston will find something interesting and different to do with the character. She's also worked on the unproduced films Wonder Woman and Brad Bird's 1906 San Francisco earthquake project.
Were there any