I also want to take a part in this challenge.
My idea is conserned not the halloween holyday, but the time after it. A bit of humor, a bit of social theme. The slogan will be "The gospel of soap and water, order in a first place".
In most cases after the big parties is a horror left on the streets - a lot of confetti, papers, bottles and other stuff. And who will clean all this? Of course, the same who has a broom!
The main idea: A morning after Halloween, street with papers, bottles and other rubish. Terrific witch bosses the broom, damning everything.
Keep the cleaness during the holydays!!!
I have also one more idea with the same character, but it will be a secret, because it has a bunch of flashing humor and I don't want to destroy a first impression.