(It's just an idea) :
Ill keep it simple:
Let people make new banners and designs for the forums.
Then the staff will choose 5 designs and post those 5 designs so everybody can vote for his favourite!
The number 1 will be our next design.
Another idea:
You could make it an event:
Every 6 months a new design will be added?
Share your thoughts about it
-Rub
(edit)
The people also need to sizes for banners...