Based on tweets and blog posts, identify new scams before they become widespread.
Is such a public system already in place? Sure, the FBI must have one, but it's not shared with the public. Also, such a scam alert system is quite similar to systems based on crowd-sourcing to detect diseases spreading around the world. The spreading mechanism, in both cases is similar: scam / disease agents use viruses to spread and contaminate. In the case of scams, computer viruses infect computers using Botnets, and turn them into spam machines (zombies) to send scam email to millions of recipients.
One way to monetize this idea:
Selling the data to central police and intelligence agencies around the world, anti-terrorist forces, or the Army, as some of this activity is used to fund terrorism. Or get it integrated with anti-virus software. Or sell to banks: they could decline financial transactions if it involves an entity listed a "dangerous scammer" by the system.
Sounds like a good idea, but it will be hard to do well. If you succeed, eBay or Amazon or Google may buy you.