Fill it to the brim with good characters.
Character. Character. Character. Listen, in this day and age of big budget films, if you want to make money you've gotta give viewers more bang for their proverbial dollar. Give them a reason to keep returning to the theater and buying more toys. Especially in these genre films. Rumor has it that Ridley Scott wanted to tease the Alien for another film, but it seems that the powers behind Alien: Covenant told him that he needed to put the Alien in THIS movie stat. Well, if a luminary like Ridley Scott can be pushed around then it seems like everybody is fair game. Which brings us to the stroke of genius in The LEGO Batman Movie. This film showcases The Joker, Batgirl, Harley Quinn, Two-Face, Bane and just about everybody else you would want to see Batman go up against. Add to this that these characters (aside from The Joker), aren't on screen for that long, and yet somehow manage to be very memorable, further bolsters the idea that this can be done. Now, I know that putting a bunch of animated LEGOs in a movie is much easier than putting in live action human beings. Still, the point should not be lost. DC might very well have something with Justice League, but did we have to watch Man of Steele to get there?
You can still go deep.
In The LEGO Batman Movie, when Batman comes home from a hard day of crime fighting, we see the Bat Cave. Surrounded by technology, cars and everything else one could want, we see how truly empty this crime fighter's life is. Then, when Batman microwaves himself a lobster dinner, we get an up-close look into the soul of this character. As the food spins and we see Batman's sad stare into his meal, we realize with certain melancholy that this is all that he has. At that moment, this kid's film graduates to the level of art. Batman hates crime and at the same time he needs it. Without it what else does he have but a lot of money and nobody to share it with. He's just Bruce Wayne and that's kinda borning. That moment, where the light from the microwave shines on his LEGO face, we get more heart and soul than we got from The Green Lantern, Man of Steel and Batman vs. Superman combined. Are you listening DC? All your films to have moments on introspection and contemplation. Don't merely edit around everything as a way to keep the story moving.