We could always bring in the Sentenials, like Tegan was previously planning behind our backs? Heh.
Or maybe the would could undergo a major demon invasion? I don't think so Tim.
Or maybe we could have a new not-so-underground organisation of evilness rise up to cause much havoc and actually succeed in partially taking over the world.
How about an alien invasion? Intoduce yet ANOTHER alien species?
I think I shall start a poll :D