Backstory: High quality designers are tough to find, someone with a 5 star rating on fiverr or upwork may not be an "elite" designer as some of these ratings and site expectations are relative. Most of the time, you get what you pay for; a $5 logo is not going to impress. Similar to toptal and upstack, a platform for high quality designer talent.
The Idea: A platform to hire premium, top tier designers.
Deep Dive: You would build a marketplace platform similar to toptal where customers can sign up and find top tier designer talent. The heavy work comes into building the designer application experience. You would have to manually vet people in the beginning and then develop a process to scale.
Validation: As stated above in the backstory, there are versions of this for developers. You could test this by first finding a bunch of top tier designers and then throwing up a simple landing page with a contact form and handle your first clients manually.
Monetization: You would charge a percentage of the overall job. You could have designers earn a higher percent as they complete more projects on the platform to incentivize them to stay.