You're right. If every piece had to follow a theme, it would be contradicting with the collab's already established concept. I'm going to be honest. The most recent Sketch Collab was the first that I'd seen, and what really won me over was how it was a team of animators pooling together their fantasies, daydreams, and imaginations into animation. It was something...unique and interesting to watch.
But of course, this doesn't leave much entertainment value for your average person. In fact, they would likely find the concept boring. I enjoyed the Sketch Collab, because I'm a Flash fanatic that had never seen an animator's sub conscience turned into Flash animation. But, If there was a theme of say...space. The compilation of doodles won't seem so random after all.
To be honest, I'd be more willing to watch a Sketch Collab with a very basic theme. The difficult part would be choosing a theme. When we doodled around as a kid, what was it that we always drew? Me, I doodled video game characters all the time. But some people doodled guns, tanks, and airplanes with everything shooting and being shot at. Some people made comics or really well done drawings, sometimes with realistic or anime art styles. I'm sure space was a popular topic among kids who wanted to be astronauts. There's also ninjas. I love ninjas. Hey, you know what Tom Fulp was probably doodling as a kid? Prototype versions of Newground's oldest games and characters. I would SO enjoy a Newgrounds themed Sketch collab. ;)
Probable themes for future Sketch Collabs:
-Video game characters
-stick figures, tanks, and airplanes shooting guns for no reason
-space
-NINJAS
-Newgrounds!
-Love (which would make a great Valentine's Day submission)
-sports
JPB
The theme should be maybe "drawings coming alive", that way, it should be a bit less random, yet keep the original soul of the collab... not really sure though, I wouldn't see much problem in having no theme at all...