Okay, so here's the basics:
Check out the shop above where your post will be, and in their medium, give them a Halloweenish challenge. For example, I make hair bows, and the next poster's challenge to me could be "Make adorable Frankenstein bows".
We can post results photos in the "new Halloween items" thread. I'll post twice and do 2 challenges just for fairness sake, as I can't inspire anybody as the first poster.