I am teaching an accelerated class of Year 10 students (14 - 15
year olds) and our theme is a Science Fiction/Fantasy.
Each student has selected their own choice of Sci-Fi/Fantasy novel/short
story to base a board game design on.  
Their first job is to make notes as they read (mind map form is great
for this) under the headings of character, setting, landmarks of the
journey/events, goal/treasure to be attained.
From there they design a proposal for their game - this must include :
1.      Name of the game
2.      Playing pieces (including any cards or devices accompanying it)
3.      Rules
4.      Board design
5.      How the game is to be played

Once the students have written these notes out fairly fully, they then
draft a layout for what the cover of this game will look like. (Front of
box only)  This will then be labelled with at least three visual and
verbal features they intend to include and the effect they want these
features to have.  i.e. use of trendy lettering to attract teenage
buyers.
Once I have discussed this proposal with the student and we are both
happy with any changes, additions or compromises that need to be made
then the final production can begin.
I imagine that I will use this approach with other themes in the future.
The cover of the actual book could be used as a starting point for ideas
for any students who are struggling to get a start.