The following three ideas can lead to passive income using Upwork or Fiverr:
Hire experts in their niches on Upwork or Fiverr to create online courses and sell them on Udemy
Hire experts in their niches on Upwork or Fiverr to create ebooks and sell them on Amazon
Hire freelance developers on Upwork or Fiverr to create software products or websites and sell them via the web
You see that you should not only enter these growing freelancing marketplaces as a freelance developer, but also as a client. Essentially, you want to become an arbitrageur of skill and work. This way, you can benefit from the increasing efficiencies of scale offered by these platforms.