Black Cat won't be the only villain, but I do agree if her purpose is to tempt, than she has no business in this film as Peter is not developing. Though Gwen I don't think tempted Peter so much as Peter used her cause MJ saw it that way, but we still covered the Peter and another girl ground. I think, while this is a rumor, it is one with merit. I think Black Cat at this point can be chalked up as in the movie. The only questionable part (in regard to the story) is if McAdams plays her to me. I think if she is up for it, it's probably hers to lose since she is gaining steam. But, I can also see her not ending up with it for a variety of reasons.
Plus, there is the possibility she is an ally, which would make this another crowded movie. Thus, I think that would be a mistake. Less is more, so please just have Peter and 2 villains. Not 2 villains and a buddy.
I'm more interested to see if Vulture is the other villain right now. That would validate that one story's plot outline for the film and we'd have somewhat of an idea of what to expect.