Sadly, the new movie titled "Legion" coming out later this month is not about the Legion of Super-Heroes, but the coming apocalypse (via IMDB):
An out-of-the-way diner becomes the unlikely battleground for the survival of the human race. When God loses faith in humankind, he sends his legion of angels to bring on the Apocalypse. Humanity's only hope lies in a group of strangers trapped in a desert diner with the Archangel Michael.
Now, leaving that all behind, let's suppose a live-action Legion of Super-Heroes movie were to be greenlit. Leaving out casting calls, what would your movie's plot be? Of course, since you're a big Hollywood writer who is not bound by comics continuity, your movie can pick and choose from among any and all continuities over the last 50 years (up to and including Mordru on a jet-ski).* Give me a paragraph.
In flashback, the Legion's origin is recounted as part of an anniversary celebration - we can get that out of the way and show that other Legionnaires have joined since then. Flashbacks show various failed applicants (from Spider-Girl and Double-Header to Plaid Lad and Stone Boy) and previous cases/missions (to show the United Planets and the world of the 31st century). One important flashback is how Superboy joined the Legion and that he's a member because he has found a group of his peers, which he does not have in his own time. In the present, the Legion comes back from a battle with the Khunds - they have taken some casualties (deaths and injuries) and they need to recruit some more members. Then we adapt Jim Shooter's Karate Kid/Projectra/Ferro Lad/Nemesis Kid story from Adventure 346 - 347. The movie ends with Nemesis Kid fleeing and being recruited some of the other rejects we saw earlier (in a method that recalls Superboy's recruitment by the Legion), they're building a new group opposed to the Legion, setting the stage for the sequel: the Legion vs a group of super-villains bent on revenge and raising hell.
*Bonus points for anyone whose movie includes Mordru on a jet-ski.