What about something like make a bad map good? If it's already been done, ignore this:
You would have and experienced map creator make a terrible map. Leave some of it with unfinished parts, orange textures, lacking detail, broken clipping, leaks, completely unbalanced etc.
From there contestants would be able to access feedback that play testers have given the god awful map. They download, and fix it. Maybe every map gets a single public play test. Not one match, but enough for people to give them feedback so far. They hand the final map in, it's played and inspected and the winner is the one who gets the highest overall points out of different sections (flow, clipping etc)
That would test different aspects of map making such as understanding the technical aspect of maps and the actual gameplay elements.