I'm expecting to be surprised. If that's possible. The scope of things that you could submit is huge, but the actual thing you need to make can be quite small, laser focused.
I'm expecting people to make awesome looking maps like things we've never seen before in the Source engine. Using the volumetric lighting, bloom, dynamic reflections. All that good stuff.
I'm expecting people to create scenes in the scene editor, by arranging models, setting up lighting, some ambient sound, and cool particles, all to make something very special.
I'd expect people to create verlet systems, liquid systems and other dynamic physics systems to make really interesting interactions.
I'd think someone would be able to decypher the shader system to make things impressive and beautiful.
I can imagine someone using some open web api to stream in data that they could then use to create world geometry and textures on the fly, creating a procedural world.