I was thinking kind of what Living LEGO said, two different challenges.
I think it might work well if we had challenges like Kieren said, ever three months, but also had a separate one each month. So the quarterly challenge could be something a bit more ambitious like recreating a movie scene, making a superhero origin story, remaking one of your previous films (weird one there), you get the idea. Not that any of those wouldn't work for the monthly challenges too, but I think we could have the ones every three months be fairly large projects and that could work out nicely.
Then every month (maybe in the summer we could shorten it to every 2 or 3 weeks if people have more time) we would also have the more 'normal' challenges just like we've been doing (but with a bit more of a defined timeline).
Also, I think that rather than mostly 'actions' like we've had for the animation challenge I think it would be cool if there were a few more opportunities by genre. Like maybe someone does a superhero challenge, or a horror challenge, action, spy, sci-fi, fantasy, etc.
I'm not entirely sure where I was going with all that, I guess those are just my thoughts on the situation.