Just throwing a few thoughts on the topic:
My first idea reading Wave's post was to celebrate cast's birthdays on the prompt of the week. Since we know the dates it would allow time to prepare contributions.
I'm not sure that with the Forum's current "population" we can have many special events. We already have several ongoing, everyday things (prompt of the week, haiku chain, etc...), so maybe we should, for the moment, play small and focus on two or three events, that would be yoinktober, advent calendar and midsummer/midwinter.
On the other hand it's well known that increasing supply frequently bolsters demand, because members would have more chance to find something that inspires them to participate.