DC and Warner Bros. are playing catch-up when it comes to building their movie universes. Both Disney/Marvel and Fox/Marvel have successfully established money making powerhouses with the Avengers and the X-Men respectively whereas DC/WB are just putting the foundations in place, with a slate to build on that which begins with Batman v. Superman: Dawn of Justice.
However, I'm not here for the stupid debates and flame-wars. I'm here because I love movies, and I love superheroes, so I want to see all intersections between the two succeed. Which means that I'm gonna give DC a helping hand. Because there's one big problem that's developing with the superhero universes on the big screen. Remember the Amazing Spider-Man 2? Or have you managed to burn it from your memory yet? If so then I apologize for reminding you of it, but it was necessary. See, the overwhelming problem with that movie was that it was essentially one massive trailer for all the movies that Sony wanted to make in the future of their now-extinct-but-maybe-not-really-extinct spidey-verse. Hence the movie just didn't sit well regardless of how you looked at it.
But that was just one film right? Sadly, no. In fact, it's a problem that I even had with Avengers: Age of Ultron. Now, I won't spoil it in any way, but it spends an overwhelming amount of time setting up the films in phase 3. Even though it's done significantly (like, times infinity) better than it was in TASM 2, and the movie is still thoroughly enjoyable despite it, it's a problem nonetheless. One that threatens to be the undoing of the whole tentpole of superhero universes if it doesn't get attended to.
But while yes, DC is behind on this front of world-building (although I ain't complaining, they spend their time on the Dark Knight trilogy after all), this gives them an opportunity to improve on the formulas that are already in place. So the one thing that I would heartily recommend (I know you're watching Zack Snyder) is to do away with the whole "phases" thing that Marvel has going on. Borrow from the X-Men in the sense of having one streamlined story that goes from movie to movie without slowing up. Whether it's an undercurrent of an endgame, or simply the whole plot of your universe as one plot spanning dozens of movies. Don't have Cyborg's movie be completely disconnected to Flash's, make them part of the same story. In fact, here's the big hint that I'm playing as my joker (hehe).
Don't do solo movies
At the beginning of the Avengers universe it was awesome to see all of the different heroes established and then channeled into one massive movies in Avengers Assemble. What wasn't so awesome, and has raised some issues, was their subsequent solo films. Sure, Guardians of the Galaxy and Captain Aerica: The Winter Soldier got over the limitations of their universes by going for more grounded personal stories as their thematic core. But Iron Man 3 and Thor: The Dark World? They suffered, like a lot.
So why not make that Aquaman film Throne of Atlantis, a story about Atlantis invading Earth and the JLA and Aquaman working to fix the situation? Make the Flash movie Flashpoint, where Barry Allen tries to save his mother and subsequently creates an alternate timeline where all the worlds heroes are twisted and at war. Make the Green Lantern movie Blackest Night, where dead heroes are resurrected and everyone has to unite behind the different Lantern corps in order to save all living beings. And so on and so forth, you get the idea. What all these stories have is that they put a singular hero in the spotlight while still making it an event important to the wider universe.
What this does for them is to create a singular universe that goes off in all kinds of directions, but which still sticks around a core group of characters that don't randomly vanish between films only to crop back up randomly. It also keeps the Justice League movies as events, but ones which get to focus more on the villains. Meaning that Darkseid, Braniac, Vandal Savage et al will all get a lot more focus in those films and become much more memorable villains than the vast majority of Marvel's so far.
So what I'm basically saying is focus on characters and story more than franchise building, but don't lose the sense of an interconnected universe which is, after all, the big grab of this kind of thing.
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