Maybe 'monthly themes' could incorporate a month dedicated to a particular challenge/contest already present on WC, and then actual themes less often. It would also advertise other WC contests a lot more.

So say for instance:

Jan: (Theme) The Frozen Wastes
Feb: 5 Words Challenge
Mar: Unlazy Poets
April: Drabbles
May: (Theme) Forest Exploration

And so on...

Perhaps contests that are 'featured' as 'monthly themes' could be a little more developed to cater for a month of challenges, for instance 5 words could have four parts, a contest a week, bring to a final for the last week in the month. (Things like that would be decided by the contest owner of course)