I second the Motion of a Same Kit/ Same Type of vehicle Build offs. Pick either a Brand Kit which has generated a lot of excitement, or a common kit that most folks might already have.
On subject matter the sky is the Limit!
Police Cars, Dump Trucks, Blue Cars, Chevrolet Vehicles........
My favorite, Flying Cars!
Some of this already happens in the Group Build Forums, but maybe promote it more. A Banner on the top of the page when it Loads? "This Years Group Build". Higher visibility might promote more participation, and encourage more builders. As it is now, it's buried pretty deep. It should be the third line on the home page, just under the "How to use the board" entry.
And no more than two builds going on at once.