If you loved Future Boy Conan, try The Castle of Cagliostro
A bridge between a film you've already seen and one most people haven't. Here's what they share, and what the second one does that the first one doesn't.
What they share
Both films are directed by Hayao Miyazaki, and they both carry the foreign gem mood tag, and they sit in Adventure / Animation territory. If that's the register that drew you to Future Boy Conan, the second film will land in a comparable space — through a different lens.
foreign gem
What The Castle of Cagliostro is
Lupin the Third gets stuck with fake loot. He and Jigen target the Castle of Cagliostro. It's a heist with a princess.

