I'd personally do an adaptation of Grant Morrison's opening JLA arc with the White Martians.
Good idea, except that it has already been done in Justice League TAS.
Which maybe a few thousand people saw. While it's true more people have probably seen the JL cartoons, they are still a smaller number than go to movies like The Avengers, Superman, etc. If the WB wanted to do this story, they wouldn't be discouraged because it was done as a cartoon once.
It's a great idea. It's a great way to introduce J'onn, who is probably the least familiar to the general audience.
But, the question is, is an invasion story like this to much like the Avengers?