Militarily, that isn't going to happen. Pakistan is not going to be capable of carrying that off. I would rather think that after Iran's government is removed, they will start to police the area. Pakistan needs to worry about that, though I don't think they will accept the idea that Iran could do it, because they are currently never going to do that.
Iran is a threat to Pakistan, but the people aren't in place yet. Southern Afghanistan as well would be policed by the Iranians.