You're right, writing would be key to this. But as you've pointed out, X-Men was done successfully even with the alterations they made. Fans are forgiving of things like changes to some costumes (Superman and Wonder Woman's can't change too much, they are iconic of the character. Batman's has changed a few times over the years in the comic books.) We'll see how much the film industry starts to push writing over effects when Spiderman comes out this year, and the Hulk comes out next year.
But I don't think it would happen because with any superhero movie, you have to set up the origin of the superhero. With only Batman being done in the last 10 years and nothing for most of the rest (Okay, Superman had Lois and Clark and currently has Smallville) that would be too much background to cover in a single movie. If they ever did attempt a JL movie, I think they would do a few movies for at least Superman and Wonder Woman first.