The app market finding problem differs depending on how you intend to monetize it. Running an app means more to an app that makes it money via advertising then one which is sold. Making money from analytics is different from targeted advertising and from having a paid app.

But if you want an interesting take on it, how about figuring out which people who are downloading the popular apps first, and tracking their behavior as a cohort. Use the trendsetters to predict the trends.