And I rather doubt that Shang-Chi will ever be part of the Defenders, if that were to make a comeback.
I don't think the Defenders will be a thing, though with Daredevil getting another season on Disney+, I think we can expect to see some of the others back in action soon, too, between Daredevil and Echo putting New York City street level heroes back on the map.
But, rather the point was that the iconic type of the two characters in the MCU is so similar (power set, secret city, conflict with an Asian-based criminal organization, connection with dragons) that it is pretty clear that Shang-Chi was set up to take the Iron Fist's slot anywhere they really wanted to use Iron Fist. And Simu Liu is not above doing TV work.