I can't specifically think of a theme, but how about having challenges where everyone has to use a specific product like 3-D Decoupage, etc. One would still have a choice of their own design/colour of the product. Similarly, you could have a challenge where everyone would have to use specific size/shape of a card - like stepper card, etc.
How about floral, fish or farm animals for card design challenge?
Some more ideas perhaps for the future - Queens Diamond Jubilee and 2012 Olympics.