The Justice League is an enormous, but relevant concept. The themes involved are timeless and very accessible, and there are all kinds of angles that can be explored, and an insane amount of possibilities. However, I really think the JLA franchise should be skewed toward "classic", but not in such a way that it prevents or interrupts successful solo franchises from being able to show key moments of a character's mythology. I envision three films, a building story, and an escalating series of events and character development issues.
THE JUSTICE LEAGUE would open with the league already existing, consisting of Superman, Diana, J'onn, The Flash (Wally West) and Green Lantern (Kyle Rayner). Eel O'Brian would be introduced in the movie as a sort of super-intern, a metahuman who helps the league to avoid a prison sentence after being transmuted during an attempted heist. Physicist Ray Palmer would have a role as the league's resident scientist, and Bruce Wayne/The Batman would feature as a reluctant former ally. Arthur would have a role as well, mostly political, to indicate the way that the world's feelings toward the league and metahumans were evolving.
The initial movie would feature the public losing their once unshakeable faith in their Justice League due to a mistake on the league's part while trying to save lives, and humanity beginning to see the flaws in the league's methods. The government, specifically The President and his Secretary of Defense Amanda Waller, would distance themselves from the league. Darkseid would be the villain, and his invasion of Earth the main conflict. Darkseid's minions Kalibak and Desaad would have key roles, and his long-lost son Orion would feature as well, in a movie that utilizes elements of The New Gods to introduce Darkseid, a credible alien threat the league cannot just defeat that causes the world to re-evaluate it's feelings about metahumans and aliens. The movie would end with the league realizing that the league must be ready for future threats, and considering enlarging it's roster.
The second film in the franchise, THE JUSTICE LEAGUE: ECLIPSE would begin with the world, leery after the events of the previous movie, moving in a new direction, less willing to trust those with great power. The league would fight the alien terrorist Despero in the beginning and briefly become controlled by him, leading to widespread alien paranoia. There would be a thread of "aliens among us" throughout the movie, and this would serve as the introduction of Thanagarian bounty hunters Katar and Shayera Hol. The main plot of the movie would involve an escalation of tensions between humans and metahumans, with United Nations chairperson Vandal Savage attempting to broker peace along with King Arthur of Atlantis and Diana, the fledgling ambassador from the league. At the same time, the government would form the Checkmate organization as a response to rogue metahuman activity, enlisting the design and scientific aid of the condemned felon Lex Luthor. Vandal Savage would manipulate Lex Luthor into freeing Despero, and using Despero against J'onn J'onnz, turn several key leaguers rogue. The rogue leaguers would be opposed by Diana, Batman and his group of new recruits: Oliver Queen and Dinah Lance. and the aid of the Thanagarian bounty hunters who had been secretly on Earth, trying to locate and contain Despero amidst spreading alien paranoia. The Justice League would end up fighting it's own, as well as many liberated metahuman felons (familiar "classic" villains from the DCU). But by the end of the movie, the damage would have been done, and the league would have been forced to disband as the world began to fear their power.
The third movie in the franchise, THE JUSTICE LEAGUE: CRISIS would feature a world where, after the events of the previous film, rebellious metahumans were out of control, a la KINGDOM COME, and the United States government's reaction was swift: a full force unleashing of the Checkmate initiative. Several private businessman would be working behind the scenes to bring back The Justice League in some fashion (Ted Kord and Max Lord) despite political pressures on metahumans. In the midst of this chaotic political turmoil, the disbanded and disenchanted heroes would be alerted to a threat of cosmic proportions, the arrival of a mysterious destructive entropic force that was sweeping through the Universe, destroying entire planets. The league would have to leave Earth and travel to Thanagar, New Genesis and several other worlds in an attempt to stop this destructive force from consuming the Earth even as tensions between humans and metahumans escalated to fever pitch. The movie would climax in a massive battle aboard the mysterious villain's starship, several key sacrifices in the defense of The Earth, and a rebirth of the Justice League itself, complete with various global embassies and oversight committees across the world.
The main players in the franchise would be Superman, Diana, J'onn, The Flash, Green Lantern, Batman, Aquaman and Plastic Man, each who would have a relevant three-film character development arc, with supporting roles for Green Arrow, Black Canary, Hawkman and Hawkgirl, and minor roles for Black Lightning and Firestorm, as well as appearances from Snapper Carr, Ted Kord, Maxwell Lord, and various DCU metahumans.