Make it almost like a Minutemen/JSA combo. A World War 2 film, maybe in 2 parts.
Have Wildcat, Mr Terrific, Zatara, The Sandman and Hourman as the main JSA.
Make it initially ambiguous if Zatara is a fake, then delve into the fact that Homo Magi's are real and account for some of the ancient myths. That Hourman is a gifted chemist who finds his super steroid solution potentially 'venomous'.....
The first film shows the formation and character building of the JSA, but it is setting the foundations. Let them become aware of Circe joining forces with the Nazi's and Zatara suggesting they elicit help from Hippolyta. See Diana as a child on Paradise Island.
Make reference to a convent, that is being broken 'If magic is allowed unchecked, what next, the Oans becoming aware? The forces of Apokolips'? This cannot happen until the prophecy.
Then a 'young' Batman film, similar to Batman begins, he trains under the League of Assassins with Oliver Queen and continues his training with boxing from a Ted Grant and escapology and occult knowledge from Zatara (where he meets his granddaughter Zatanna).
Then a Superman film, let Clark remain hidden with his powers, meeting Luthor, where he finds Lex has appropriated alien technology from his rocket to create weapons not suited for Earth, including a war suit. His head of the toy department, meanwhile has been creating toys to terroise Metropolis. The police can't handle this, so eventually Superman steps up take down the Toyman. Luthor congratulates him, whilst Superman suspects Luthor's involvement. Or maybe show Luthor has a second shuttle from Krypton, with Supergirl in it. Make a big show of Superman being the 1st public superhero.
Post Credits - In Paradise Island, have a shocked Hippolyta say 'it is now time, the covenant has been broken'. An adult Wonder Woman prepares to leave the Island.
WW film - Have Wonder Woman prepare to integrate herself with the main world, flashbacks to her training and relationship with her mother. Have Zatanna explain Circe is back on the scene, but her magic can't counter a goddess. The 2 team up in, with Zatanna as her guide to the new world and Steve Trevor as her 'escort' from the UN, who can't get his head around aliens and magic. WW defeats Circe at the end, with reports that she had agreed to work for a place called 'Apokolips'.
JLA - The arrival of Brainiac. Brainiac has been sent to 'collect' samples from the earth 'before it's destruction, now the conveant has been broken'. Superman destroys a probe, until he is hurt by some of Brainiac's weapons (kryptonite based). Luthorcorp grabs the damaged probe.
Brainiac then attacks Paradise Island and takes a chunk of the land and alongside a portion of New York.
Get a JLA team to counter Brainiac. Batman, Superman, Wonder Woman meet, then get through a rough fight. They realise that Wonder Woman has mystical arrows, which Batman can provide technological arrows which the best sharpshooter in the world can use....Green Arrow.
Olliver meanwhile is p!ssed. His super hot mma girlfriend was in New York and he's geared up. Wonder Woman collects Zatanna to help.
Meanwhile Brainiac is messing around doing experiments and has been simulating human organs to match some of the beings he has encountered on Paradise Island, he successfully creates a hybrid of Ollie's girlfriend and the Silver Banshee, the world's first meta human....Black Canary.
Luthor and his secret weapon, a blonde girl whose powers match Superman's then attack Brainiac with Superman's posse. They successfully fight him off, with Brainiac muttering it is better to leave now before the Oans or Apokolips turns up....
We are then left with a self proclaimed leader of Lex Luthor getting the glory with his 'Justice League', Batman, Superman, Wonder Woman, Zatanna, Green Arrow, Black Canary and Supergirl.
Batman's second film is about Bane who has been able to get Hourman's formula, whilst Joker is aggrevating both to set up his own agenda ...and he see's right through Lex's persona.
A film with Superman exploring his relationship with Supergirl and they are transported directly to Apokolips. They encounter a New God party trying to negotiate a truce. Superman is disgusted by the the politics of Apokolopis, but both him and Supergirl are completely outclassed by Darkseid (who is largely absent from the film), although they do defeat Kalibak and a couple of the furies in a previous fight. Orion sends them back and they completely understand the dread going back. Superman's arrival has brought Earth back into 'fair game' as it has reached 'first contact' with another race.
Maybe a film about Thangar, establish an Olympic vs Egyptian rivalry. Then a Green Lantern film.