It would be a long movie. That's why its absolutely necessary they hire the right director, cast and get the best script so they can get the best quality and not waste time.
Putting Bale's Batman in would make him the focal point for viewers because he is the most familiar. This, in result, would keep viewers from getting confused and trying to focus on Batman, Superman, Wonder Woman, Flash, Green Lantern, Aquaman, and Martian Manhunter all at once.
That's the beauty of ensemble films. The viewer will only see what the film makers want them to. They can give some higher screen time and others less when the story requires it.
Having solo films would allow them to do this with the more established characters so they'll still know the basics without having to catch up in the film. They can just focus on the story then.
Batman doesn't need to have that role. It could any character. Martian Manhunter would make sense to introduce in JL while the primary characters are already established in solo films.
This can allow the viewers not only to get to know him but to see how a new character they don't know much about interacts with the rest of them.
I'd argue that they could just use one main character through the film and get a good result, as well.
Just my opinion.
That's if it succeeds. If it didn't they just killed off about 6 franchises before they got started. It would be decades, if ever, they could be revived again in film, cartoon or tv shows.
Thank god WB is using the Marvel method now.