For the first Justice League movie.
A) What kind of story do you want? Who is the villain, what´s his goal why chose him (because superman can´t handle him, he´s classical JL villain etc).
One wish though. Try not to linger on how JL 1 2 3 will be connected. Just how would you like the first movie to be without using super plots stretching over 2-3 movies.
B) How to properly connect the story´s heroes before or in the movie (Batman and Superman will know eachother but it might make sense that other heroes are connected) and how to make them unite and start/join the JL?
I'm going to be honest, I want another alien invasion. I love the fact that MOS was sci-fi and I want them to stay with that theme.
Here's the thing about the forming of the Justice League. Somebody has to have the vision. They can't just come together "just because". Somebody has to know there are people like Wonder Woman and Flash out there, and they have to have the idea to bring them together.
I think it should be Martian Manhunter.
I love sci-fi, and I think they should delve deeper into the genre and introduce time travel. I like the idea of him coming to Earth because of a device a dying fellow alien[killed by Darkseid] gave him that would allow him to time travel as a one time thing, "see for yourself".
It's a future where the Earth lies in ruins, your typical post-apocalyptic stuff. He learns that it's because a tyrant named Darkseid invaded the Earth and took advantage of the heroes never coming together to form a team to be able to stop him. Maybe in this future he meets up with a Future-Batman, who tells him this, "Heroes from around the world arose to try and stop him, he killed them one by one. We all fought alone. If we had joined together and fought as one, we could have saved humanity. That was our mistake." Throw in something like "Kal was the first to go, then Diana, then Barry" while showing Darkseid killing each one of them to explain what happened to everybody.
This leads him to Earth where he finds all the heroes and warns them that they have to join forces or else the Earth is doomed. He first goes to Batman, who then contacts Superman and they set out to find Wonder Woman and Flash and form the Justice League.
I think Darkseid's motivations for coming to Earth should be for its resources(yeah I know).
I think this can go down in a lot of ways. It can happen like above where MM is able to travel to the future as a one time thing where he sees the destruction of Earth. Or you can have Future-Batman build a time travel device where he sends a message to his past-self about what happened and how he needs to bring the heroes together. They can accomplish it in a lot of ways.