If you loved Batman: The Dark Knight Returns, Part 2, try Batman: The Dark Knight Returns
A bridge between a film you've already seen and one most people haven't. Batman: The Dark Knight Returns has roughly 10.0× 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

Batman: The Dark Knight Returns
What they share
Both films are directed by Jay Oliva, and they sit in Action / Animation / Mystery / 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 Batman: The Dark Knight Returns is
Gotham City skyline, winter, a shattered bat signal flickers. An aging man in a decaying costume stalks alleyways where neon reflects off rain and blade steel. This is Frank Miller by way of 1980s VHS cover art — practical, grim, and aware of its own myth.