Mabel...is really one of the most powerful characters on the show. It seems like she always helps important plot points happen, whether they be episodic or for the overall story arc
FORESHADOWING Hah! I don't know whether the writers did this on purpose, but whether or not they did, that's awesome! (Heh. And now even "Princess Unattainabelle" has a deeper, hidden meaning. Curse you, Gravity Falls)!
Bill was killed in Stan mind, so, if he come back.... Stan! Oh No!