1) Best hero system that can be imported to any new map.
2) Best Custom Battle System/core game mechanic that can be imported to any new map. (Recently I've seen reaper arena, some FPS games, 2 Turn-based combat systems, Final Fantasy combat system, card games.)
3) Best custom building model
4) Best custom texture tileset
5) Best marine impersonation (voice acting with new marine one-liners - entries have to be serious one liners, not just a bunch of dirty jokes in a marine voice)
Category 2: Best "discovery".
1) Best method for making a dialog that builds buildings.
2) Best method for keeping high scores on Bnet.
3) Best method for fading from one area of the map to another without just zooming the camera.
Category 3: Best custom map....
1) in less than 10 minutes (open up those video recording programs)
2) with less than 20 triggers.
3) made by a team of 3 or more who shared the workload equally.
How about a model attachment contest ? Have to use all ingame model and actor in game data to attach into something cool. It can be either done with trigger or data editor. Custom attachment point is acceptable. Model scale (x,y,z) is acceptable. The good thing about this is no actual modeling skill is needed ( more people can compete) and easier to compare/vote by screenshot and awesomness
Edit: It can be something like the death baneling's use of model hidden partially underground
So it needs to be fun, it needs to be done quickly and people need to learn from it.. for me, theres one thing this works with, most ppl. would learn from it and its done really quickly:
Easy as that. Galaxy edit comes with its own sound recreation tool, allowing pitching, fading, stereo resetting and all things needed to make great ingame sound remixes or funny quotes.. I did use the sound recreation tool in my reaperXzergling map and the result was pretty amazing.. its obviously not "video vegas 9" (or whatever the new tools are called) but it gets the job done, is fast and easy to use..
The contest could be about "funniest single remix of an ingame sound" or "best remixed soundset of a single unit"