Don't lead into a Justice League with The Dark Night Returns.
Start with a PG Shazam movie. Start with a street kid doing the right thing even with all the other kids being stuck up Iphone cyber bully jerks. Cast the Rock as Shazam. Mention Superman a lot, maybe have him in the movie, have superman talk about meeting up with Wonder Woman. Put a "Who is Batman poster in the background. Foreshadow Black Atom with a Wizard Flashback. Have Shazam use all 7 of his powers in smart way, take ideas from the comic book. This was once the most popular comic book take one of the popular stories and make that your act 2. Act 3 Shazam Vs Black Atom, big dumb fight. End with weeks later Superman asking Shazam to join the Justice League. Throughout have the Rock say funny kid stuff. You can't fuck this movie up, kids will love this movie.
Make a Batman movie. Batman is fucking popular, almost any story will work. Give batman a reason to not join the Justice League. But force him to join it, something along the lines that Batman has good judgement because he is a regular person. Batman movie ends with Justice League forming at the Hall of Justice created by/for Superman who is known for saving the world. Superman WW and Batman start building the team.
Make a Wonder Women movie. Have her meet up with lesser superheros from the 50s and 60s. She can fight Russain super villains, black manta or some shit tease aquaman. The movie is about revealing WW's powers and how she wants to do something larger for the world. Perfect for her to team up with Superman and Batman
A flash movie, Aquaman and green lantern won't work. Skip them.
Have green lantern get recruited by Batman. Aquaman get recruited by WW. Flash by superman, have a "Race the Flash" city Marathon and Superman comes to try his speed against the Flash.
The bad guy is Brainiac, who has an army of aliens and mechs for the JL to fight. Superman is made powerless by Cyrptonian home world stuff.