I’ve done this sort of collaborative work and it’s a great deal of fun. In one instance, myself and 5 other guys passed around a Blender file to build a scifi environment. Each day, one of us would add as much or as little as we cared to and then pass it to the next guy. The environment got quite large and very gorgeous.
The Zbrush artists for Gears of War did the same thing. They just passed around a Zbrush file and each guy built on the other’s work. (google Gears of war + Zbrush and you will be stunned). Zbrush also hosts a live stream series called “pass the sculpt”.
I suppose the problem of making a team-based contest is that you must have a competent team and hope they don’t ditch you to do something else. Still the concept is a real blast if you have enthusiastic artists and the time.
The way you are currently doing things with the Mythological Creatures contest may be the best way to do it I guess. It’s a very interesting idea to ponder.