I had a blast on Intrade during the 2012 US elections. I was worried that after those elections were complete there would be no remaining 
active markets in which I could participate (I don't enjoy sports predictions). However, I found a new home in box office predictions. At least, until Intrade went the way of the dodo.
They would typically choose popular films set to premier in the upcoming months and set markets for their premiering weekend box office sales. An example for last weekend could have been:
- Interstellar to earn over $40,000,000 during it's opening weekend
 - Interstellar to earn over $45,000,000 during it's opening weekend
 - Interstellar to earn over $50,000,000 during it's opening weekend
 
I hope you'll consider adding markets like these. Also, a formal place on the website to suggest new markets would be nice. I have tons of ideas (which you may or may not be interested in reading).
edit: They used 
http://www.boxofficemojo.com/weekend/ as their official measure of what a film earned