AS - you mention the possibility for two challenges for each two month stretch...

If the first of the challenges in each month are going to be topic driven (chosen by previous winners and based on a theme as they are now), then maybe the second challenges could be very bryce feature driven.

By this, I mean there could be a challenge to highlight hdri effects, one for boolean modelling, another to highlight the tree lab, one for the sky lab, another to highlight the dte, another the light lab, and another the terrain editor (as examples). This would also fit in really well with Zhann's  ideas about attaching a tut that describes a bit of bryce's core feature/functionality to the challenge and making one of the challenges a learning experience thus possibly making it more attractive to some beginners. 

... just a bit more dendrite-neuron-axon activity ;o)

Dreams are just nightmares on prozac...
Digital WasteLanD