As much as I hate to say it, I think more people will want art than fan fiction. It might be better to do separate ones, but then i don't know if the novelists will want to join in.
The only other option might be people don't get a choice what they get- they get what the other person can do. If everyone knows this going in, there shouldn't be complaints. It would mean tho, that people couldn't be very specific about what they wanted other than characters they would like to see included and any basic notes on how they get along.
For getting more people to sign up- if Tapas is supporting this, then maybe next year they can pin the thread. I only saw it because it happened to get bumped after we started the comic and became active on the forum. Otherwise I never would have known it was happening.
As a side note, a summer event could be fun but messy. People often are busy during the summer and off doing things. It's easy to lose track of everything.