A group of interested participants could focus on a theme or topic (concept) project, for example, at some time in the future PorBUG may want a set of PorBUG identifying artwork (logos, emblems, full-color, black-and-white grey tones, etc).
A Concept Collaboration may have different stages. The first may be each collaborator forming their own base modeling concepts independently. Then the group could judge the most popular (Contest Winners!).
The group could then pick and choose all the choice parts from all the entries to determine a final foundation before the next stages; materials and texturing, lighting, compositing, etc with winners and picking and choosing at each stage.