Before anyone flames me for not reading comics, as they are a valid art form, I do read them, just more stuff like Preacher and Y the Last Man.

here's how you set it up first 45 minutes or so

ext shot, night, Gotham: Batman is crouching on a garagoyle looking over the city, he sees someone running, he ziplines or swings or whatever down, a petty criminal from all apperances but he was actually stealing a conisior that clearly says Wayne labs

scene 2 Wayne Manor, Bruce and Alfred are working on something, recieve a message from Lucius Fox, Wayne Labs and Star Labs have been working togather on a new protype spaceship or fuel or something, Bruce turns to Alfred "looks like we're going to Metropolis"

scene 3 Metropolis: Lois comes over to Clark with information the Wayne is coming to Metorpolis, Perry wants them on it, blah blah blah

cut to Star labs, Clark and Lois are there with other reporters, Lois is being annoying trying to catch Bruce's attention, there's an attack, Bruce and Clark run off, Supes and Bats appear a little banter "what are you doing here" "investigating this" however the ailen attack is too much, supes to Bats "we're going to need help" Bats to supes "you get the help, I'm going to investigate this showing the canister and some alien spaceships they destroyed, Bats goes off to play dective, Supes goes recuits, there's another battle with Supes, Flash, Aquaman, WW but again butts are kicked because of lack of teamwork, GL comes down last minute and saves them, Flash is like "who is this a little banter, GL goes through a GL speech explaining who he is blah blah blah

an explantion of an ailen threat (or if you go with Jonn jonez this can be done telpathically with scenes of the ailen homeworld) Batman shows up confimrs what GL just said, and he's figured out what can stop them. Big ailen fight, Bats in the Batwing shooting down spacecraft, supes on ground, land sea, GL whipping out crap from his ring, wonder woman punching stuff, Flashy running around, Aquamna I don't know have burts of water shooting out from the ocean busting crap up, short circuting ailens, there you go JLA movie

as much as I would like to see certain team up story arcs, get real. these movies are not going to produce a more complicated super hero world.

I guess I can see how you might think that simply because of the necessity to maintain continuity. but I should also point out that there really isn't any continuity in the comics themselves (except for crossover arcs). and I would expect FAR less time dedicated to character development for a team of superheroes than for one hero and one or two villains. so it's exciting but also scary.