Justice League doesn't have to be fantastic, though. He can do naturalistic with superpowers just as well as you can do it with rich people in bat costumes. You've just got to be willing to take it seriously and follow through to its natural conclusion.
That's a beautiful fan theory, but it's yet to be demonstrated.
Marvel, of course, has succeeded admirably by wrapping fantastic stories and action around plausibly likeable characterizations by good actors.