Some sort of user quality measurement / recommendation engine thingy.
There's a lot of bullshit on Twitter these days, we need tools to sift through it to find the good stuff. Not Twitter's "Suggested Users" list, which mostly contains celebrities and companies.
There's a lot of metrics you could use:
* negative points for "follow-baiting", i.e. following hundreds or thousands of random users, hoping they'll follow you back, then unfollowing the people who don't follow you.
* number of user's Tweets favorited (normalized by number of followers?)
* average follower's rating. kind of like PageRank.
These are just a few off the top of my head.