After Warner Bros and DC plotted and planned after the release of Man of Steel, a decision was made along the way to throw Batman in there and see if they could get the whole Justice League thing started. And everyone said the same thing. "Marvel spent 4 years building up a universe before they did Avengers, to jump from Man of Steel to something called Dawn of Justice is rushing it."
The one point that everyone forgets is that ensemble movies are a thing. To complain that 6 superheroes is too many to cram in to a movie with no build up makes sense until someone points out that Bryan Singer did it years ago with X-men. And the reason Avengers worked wasn't just because of the build up it was because Marvel brought in Joss Whedon, someone who knows how to juggle a large cast of characters and make it work really really well.
If DC was so desperate to make a Justice League movie they should have skipped the sort-of sequel that is Batman v Superman and just gone straight to the main event. This actually almost happened when a Justice League movie was rumoured for 2015. But I guess after Man of Steel did okay, it was easier to just continue with the same team and make a sequel instead.
Once you start to pull away from the idea they should just "do a Marvel" it opens up all sorts of possibilities. Imagine if *slight spoiler* instead of Bruce finding clips of the meta-humans on a dropbox folder, he actually found that the Justice League already existed.
That would have been one amazing bait-and-switch for a film called Batman v Superman. Have the two characters at each others throats but then discover that in another corner of the universe the League was already getting shit done.
Copying Marvel might work if the right filmmakers are involved but studios need to stop aiming for the "connected universe idea" when they'd find way more success by just making a damn good movie first. And unfortunately that hasn't happened yet.