If you loved Batman: The Dark Knight Returns, Part 2, try Next Avengers: Heroes of Tomorrow
A bridge between a film you've already seen and one most people haven't. Next Avengers: Heroes of Tomorrow has roughly 5.2× fewer votes than Batman: The Dark Knight Returns, Part 2 — it's a deeper cut, not a mainstream recommendation. Here's what they share, and what the second one does that the first one doesn't.

Batman: The Dark Knight Returns, Part 2

Next Avengers: Heroes of Tomorrow
What they share
Both films are directed by Jay Oliva, and they sit in Action / Animation / Science Fiction territory. If that's the register that drew you to Batman: The Dark Knight Returns, Part 2, the second film will land in a comparable space — through a different lens.
What Next Avengers: Heroes of Tomorrow is
The next generation of superheroes trades hand-me-down capes for laser blasters. After their parents' fall, the Avengers' kids regroup, powers flaring, to face the villain who started it all. A respectful bow to legacy without much originality to call its own.