Backstory: A friend of mine recently was rated a top developer on upwork from all of his positive reviews. Now he has an abundance of work, what happens when he leaves upwork or he falls off the list?
The Idea: A 3rd party review site for freelancers.
Deep Dive: Similar to trust pilot for businesses, you would build a way for freelancers to post their ratings/reviews and give their customers a way to write reviews for them.
Validation: Trust pilot is a great example of how this can work for businesses.
Monetization: You would charge a small monthly fee for the service.