I've found that fandom primers are one of those kind of posts that are really fun to make and read but aren't natural for generating discussion, so I was thinking for the first challenge it could just be a really general "Make a post to prompt discussion about some fandom thing that everybody can participate in, no matter their fandom"? And maybe you just sign up for a week, and can post any day that week, the way crack_van worked - that way people can fit it into their schedule.

But if I did it I would, as with most things for that comm, go for the low-effort version of modding it that was basically just a sign-up calendar and maybe a polite reminder comment at the beginning of your week - that's what I did the last time I ran a challenge like that on a comm (which was quite some time ago, but I think I remember!)