OMG, You make so many points in one single post.
I agree that DC character have a grander scale. In fact, that is something I have said for a long time. Each of them, may it be Green Lantern or Wonder Woman or other, have a rich story to be told, that doesn't have to rely on something else to work. Just look at Superman. Can you imagine someone else interferring?
Unless they set the whole movie up as a combination with another hero, it shouldn't be done. And even then it will be quite hard. The film should not have one visual style, but TWO. The characters crave for that.
I don't know if it's right to compare with religious stories. But what if Noah, Moses and Jesus met? It would pretty much change everything, and take away the essence of each story.
And as Cain says, each DC hero shall stand out as the best we have hin the world. It's important! I say that they should all take a role of a saviour because of they power level. A little bit of a messias figure. Not only Superman is such.