Long read...
I'm imagining a Justice League movie after they've had Man of Steel, Batman's reboot, Flash and maybe Wonder Woman. The first movie could have the White Martian invasion(harvesting humans maybe?). Supes, Batman, and WW could be the first members in the movie, shortly after recruiting Flash onto the team. Lantern is asked to join as well but initially refuses because of his dedication to the Corps, only getting involved because of the invasion threatening sector 2814. J'onn could be introduced seemingly as a threat to the league at first, but they soon recruit him because he knows the Martians and their weaknesses better than anyone.
I think it would be a great way to introduce J'onn into the films without giving him his own movie right away. That way WB can know if the GA likes the character or not, and make more movies accordingly.
After the first "phase" (MoS, Flash, Justice League, etc.) WB/DC will hopefully have proven that they can do these movies properly to the GA and can move on to giving more heroes their own movies like Wonder Woman(if they haven't done her already), and Aquaman while also making a GL sequel.
I think Aquaman's movie could be a huge underwater epic, the likes of which we've never seen(If Marvel hasn't done Namor by then)! His movie could focus on his life as a husband and King of Atlantis with his brother Orm by his side, plotting to take his place as King. Black Manta could be recruited by Orm to take Arthur out so he doesn't get his "hands dirty". Manta could even "seemingly" die, only to return in an Aquaman sequel as the main villain with modified armor to keep him alive(think Vader), new weapons, an elite force of mercenaries, and a personal vendetta with Aquaman.
The end of Aquaman's movie could lead up to the events of JL 2, ending with him being wrongfully banished or shamed from his kingdom, followed by Orm taking his place as king while keeping a close eye on Arthur's son and wife just in case he ever needs to use them against him. Knowing that Orm has his wife and child under heavy watch, Aquaman looks on from the distance as he sees his people preparing for war against the "Landwalkers" above them (Orm's goal all along).
Justice League 2 could focus on the Atlantean Invasion/War, with Aquaman venturing onto land and joining up with the League to stop Orm and the Atlantean empire. Aquaman could temporarily replace GL who's too busy protecting another part of sector 2814. In the big finale GL could finally return to Earth just as the Atlanteans are getting the edge in the ongoing war, accompanied by a battalion of Thanagarian war ships. It turns out Thanagar was the planet GL was helping while in space, so they're paying him back with their assistance. Hawkwoman could be introduced, shown fighting alongside the JL against the Atlanteans and later being asked to join the team after she proves herself. After killing Orm(Now Oceanmaster) and gaining back his title as King, Aquaman and his people return to the depths after he personally thanks the JL, offering himself as an ally.
Maxwell Lord and Checkmate could also be the side enemies in JL 2. How can the population trust the JL when they've repeatedly been aligning themselves with the "enemy"? First, they let a Martian into the League and now it's one of those dangerous Atlanteans? I can imagine the League getting serious hate for their choices in the beginning.
I'd love to the foes of the JL member's solo movies returning to form their own team to take the League out(Lex, Joker, Black Manta, etc.), meanwhile Darkseid is threatening the planet with his eventual arrival, along with the rest of his Apokoliptian forces.
