Im not saying a JL movie is 100% unworkable. But it wouldnt be easy. So, yes, if the story was good
then it would be good. (And if my uncle had boobs shed be my aunt.)
In action ensembles (like Mission: Impossible) you want to give each member a specialty. So, theres the brains, the muscle, the master of disguise, etc. The pretext is that there are jobs in the caper-at-hand that are uniquely and plausibly suited to one member which no one else can do (thus justifying their inclusion on the team). And it goes without saying that youd try to avoid duplication (theres no cause to have two masters of disguise on the team, for example).
So for JL, I think the first and biggest problem is Superman. Characters like WW, Flash, Aquaman and (arguably) Batman are derivative of Supes he can do everything they can.
Now, you can rationalize the duplications on the basis that Supes cant be everywhere at once. But that seems a tad weak. In the same way that you can fashion the narrative so that a team is necessary, you can fashion it so that its not. And if there was ever a character who could handle simultaneous crises, its Supes.
You can also rationalize things by depowering Supes. He cant survive underwater or in outer space so that gives purpose to Aquaman and GL. Or
Supes isnt the sharpest knife in the drawer so bring in Batman (the brains). But taking Supes down several pegs in order to justify the presence of others already starts to smell bad.
Perhaps a JL that is sans Supes is the way to go...?
That's only a problem if you use the Superman from the 1960's, who was just a fancy deus ex machina who could pull a new power out of his arse and save the day. Superman is an all rounder character, everyone concedes to that point, but when you are an all rounder character, you have no specialities. Superman may be fast, but Flash has the Speed Force. If he wanted, he could prevent Superman from ever moving again, because through the Speed Force he essentially controls all kinetic movement in all of spacetime. Green Lantern is more powerful than Superman, his job is to protect thousands of planets from these kinds of disasters all the time, along with much greater threats, but when it comes to Earth suddenly he needs assistance. Martian Manhunter possesses more powers and more useful than Superman on an equal scale, he is at a very basic level, more powerful.
Superman is the character who is least depowered by being on the Justice League, because apart from Batman, Wonder Woman and Aquaman, he is the least powerful.
If I could recommend some reading on the Justice League I would say something by Grant Morrison. He consistently portrays Superman as the god he is, but he does so with every other character too. Here's a list of the feats he had them do:
Superman-
(Issue 3) Even while suffering from kryptonite poisoning, Superman grins to himself as Protex screams to his teammates, currently having their asses handed to them by you know who, Batman! Batman! Hes only a man!
(Issue 7) Uses his new powers to repel the moon away from the Earth, which its plummeting towards, and for an encore, wrestles an angel.
(DC One Million #4) Returns from the heart of the sun in the 853rd century, and uses the lost Green Lantern ring to crush what's left of Solaris.
(Issue #26) With most of the team downed by the Ultramarine Corps, he confronts the troops and casually absorbs every bullet they can fire, until they finally come to their senses and listen to what he has to say.
(Issue #41) Flies into Mageddons core and absorbs enough anti-sunlight to shut it down, then announces, Armageddon is canceled until further notice.
Batman-
(Issue #3) Batman discovers the truth about the Hyperclan and then starts taking them down one-by-one.
(Issue #11) Tells Robin that he knows Lex Luthor has targeted the JLA, and that Lex doesnt realize hes facing someone else very familiar with corporate takeover techniques...Bruce Wayne.
(Issue #13) Survives eight years in Desaads torture chambers, beats him, and then takes his place in Darkseids court.
(Issue #14) Tricks Metron into becoming human, knocks him cold, and pumps him full of hypnotic drugs.
(Issue #15) Places not one, but two undercover operatives inside the Injustice Gang.
(Issue #38) Takes down Prometheus and cheats to do it.
(Issue #41) Gives Superman the pep talk of his life, while attempting to rescue the Man of Steel from absolute despair deep within the bowels of Mageddon.
Martian Manhunter-
(Issue #4) Martian Manhunter punishes seventy white martians by brainwashing them into joining the human race they attempted to conquer.
(Issue #11) Enlarges the more irrational right hemisphere of his brain to navigate himself and Superman through a hard light holographic maze powered by the Jokers thoughts.
(Issue #15) Painfully stops Superman from destroying the Philosophers Stone after receiving a telepathic alarm from Green Lantern.
(Issue DC One Million #3) Takes on Vandal Savages blitz engines.
(Issue DC One Million #4) The Manhunter from Mars IS Mars.
(Issue #37) With the team being dismantled by the Injustice Gang, returns to active duty, reactivates the teams telepathic link, and announces that the counter-offensive has begun.
Green Lantern-
(Issue #7) Green Lantern and Flash construct and then power a giant tuning fork that cancels out the angels harmonic vibrations and expels them from the earthly plane.
(Issue #22) Sandman tells Kyle that he shouldnt feel inferior to Hal Jordan...that hell surpass him and that he already knows things Hal never did.
(Issue DC One Million #4) After some convincing from Batman, Kyle creates a supernova within Solaris and contains the massive explosion that results though sheer force of will.
(Issue #31) Uses color theory to convince a fifth dimensional court to intervene on behalf of the third dimension.
(Issues #40-#41) Forces his sabotaged ring to work as Mageddon approaches. Mageddon, huh? You and me, dude.
Wonder Woman-
(Issue #7) Wonder Woman trashes an angelic chariot and then catches it before it falls onto the city below, even though just touching it sets her skin on fire.
(Issue #14) Takes on both Darkseid and the monstrous Grandmother Boxx, giving the resistance enough time to complete their impossible mission.
(Issue #41) Leads the armies of man against Mageddon.
The Flash-
(Issue #3) In a race/fight around the world, Wally beats the Martian speedster Zum using a Flash Fact.
(Issue #7) Green Lantern and Flash construct and then power a giant tuning fork that cancels out the angels harmonic vibrations and expels them from the earthly plane.
(Issue DC One Million #4) Using a strategy engine from the 853rd century, Flash thinks faster than the universe's ultimate computer.
(Issue #25) Hobbled by shrapnel in his calf, he lends Aquaman a bit of the Speed Force so they can outrun the building falling down on their heads.
(Issue #40) Returns with the last survivor of Wonderworld who escaped Mageddon by running perpendicular to time.
Aquaman-
(Issue #13) One of the last members of the JLA still alive in a word conquered by Darkseid, Aquaman leads the last-ditch superhero assault on Desaads Vegas stronghold.
(Issue #17) Manually diverts the water from his personal tanks into the Watchtower after Prometheus sets a fire that burns away the stations air supply.
(Issue #23) Routes a telepathic signal from Martian Manhunter through the alien conqueror that convinces It the Earth is too toxic to be conquered and that It should run before its too late.
(Issue #41) Emerges from the ocean with the armies of Atlantis and declares that hes the sworn protectorate of over fifteen thousand submarine states.
Miscellaneous-
(Issue #5) Hitman shows up to the JLA recruitment drive for the sole purpose of checking out Wonder Woman with his x-ray vision, does it, and then immediately leaves.
(Issue #9) Green Arrow Connor Hawke saves the JLA and probably the entire world using his late fathers trick arrows.
(Issue #14) The Atom breaks into Darkseids brain and then destroys it.
(Issue #15) Aztec deactivates twelve nuclear warheads teleported onto the Watchtower in less than four minutes.
(Issue JLA 1000000) Steel leads the Justice League into battle against their highly advanced counterparts in the Justice Legion A. Later considers unleashing the secure security systems on them, but chooses instead to trust their intentions. Even later, his time engine is used to activate Solaris quantum softdrive.
(Issue DC One Million #3) The Atom figures out how to beat the Hourman Virus while broadcasting live from Oracles lymphatic system.
(Issue DC One Million #3) Huntress thinks of a way to save the future---by burying the weapons theyll need to win in the past.
(Issue DC One Million #4) The Superman of the future punches his way through the time barrier to make it back to the 853rd century.
(Issue #31) Stripped of his armor by Triumph, Steel crawls through the maintenance ducts until he reaches his laboratory and is able to step into a new suit of armor...the entire JLA Watchtower.
(Issue #39) Oracle is given a Mother Boxx from Metron, which upgrades all of her equipment and gives her digital telepathy to better coordinate the worldwide offensive against Mageddon.
(Issue #41) Zauriel dies on the Watchtower for the express purpose of returning to Heaven and rallying angelic support to defend Earth from Mageddon.
(Issue #41) Animal Man has the idea that saves the world.
(Issue #41) Aztek unleashes four-dimensional energy inside Mageddon, hurting the primordial annihilator but killing himself.