I know this is over a month old, but I would like to offer some suggestions. I'm busy with college at the moment, but I'd love to see the challenge come back.
What if it was done on a seasonal basis instead of monthly? For example if the next one started on the first day of winter (December 21nd this year, I believe) then the deadline could be one week before the first day of spring, leaving a week for voting before the next one starts. The extended time frame would also give people time to recover from unexpected fits of reality getting in the way.
Also, lets open it up a bit. Yes, this is a Trek forum, but we do sometimes see other fandoms represented in this fanfic section. Why not let them play in the challenge too? I think that would give people a lot more to work with, and possibly generate more interest.