I might join if it's hosted after about a month. Until then I'm stuck on my studies.
Also, I'm much against the open theme for the plain reason that everybody loses.
-The judges, which you already knew about, will have one hell of a job to compare completely different themes.
-The contestants themselves will have problems themselves sticking to a solid story because they have no handles to hold onto. Especially for the new cinemakers it'll be disappointing when they hear their story goes from one end to the other, which is definitely going to happen. Too much freedom is also prisoning.
-Last of all is everybody else, who can't decide who to vote for because it's all different. The results, as you might be able to predict, is a poll where they vote for the most popular name without actually giving the other cinematics a chance. We all know this happens a lot, which is why some people even want to ban public poll. As you can imagine, this is even more frustrating for the new cinemakers.
So there you have it. 3 reasons why nobody would be happy with a themeless contest. My suggestion would be a broad theme. A theme that gives direction to all 3 of these involved parties without limiting creativity. That is best achieved by making the theme focused around the viewer, not the cinematic. With that, I mean the theme's aim is bringing the viewer to a certain condition like happiness, sadness, confusion, over-awareness, etc. instead of fixing the theme on the cinematic by using themes like world war 3, a hidden chapter of WC3 or pedobear's vengeance to civil society.
Tell me what you think of making the theme something global to provide a certain handle for the cinemakers.
PS: I think if this contest is going to be approved and you need a lot of contestants, it would be a perfect moment to accept both WC3 as SC2 cinematics. I'm curious how well the SC2 ones will come out and whether or not it'll be popular. I don't think it'll be a problem judging the different games used, since all that's different is the detail (which is not a point of judging) and available code (which shouldn't be anything for cinematics)