If you loved Galaxy Express 999: The Movie, try Space Pirate Captain Harlock: Mystery Of The Arcadia
A bridge between a film you've already seen and one most people haven't. Space Pirate Captain Harlock: Mystery Of The Arcadia has roughly 3.7× fewer votes than Galaxy Express 999: The Movie — 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.

Galaxy Express 999: The Movie

Space Pirate Captain Harlock: Mystery Of The Arcadia
What they share
Both films are directed by Rintaro, and they sit in Adventure / Animation / Science Fiction territory. If that's the register that drew you to Galaxy Express 999: The Movie, the second film will land in a comparable space — through a different lens.
What Space Pirate Captain Harlock: Mystery Of The Arcadia is
You captain a derelict warship in deep space, marooned with a skeleton crew and a cargo of ghostly secrets. Then the ship’s AI starts whispering coordinates that rewrite themselves.