If you loved Little Nemo: Adventures in Slumberland, try Sea Prince and the Fire Child
A bridge between a film you've already seen and one most people haven't. Sea Prince and the Fire Child has roughly 10.0× fewer votes than Little Nemo: Adventures in Slumberland — 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.

Little Nemo: Adventures in Slumberland

Sea Prince and the Fire Child
What they share
Both films are directed by Masami Hata, and they both carry the playful mood tag, and they sit in Animation / Family / Fantasy territory. If that's the register that drew you to Little Nemo: Adventures in Slumberland, the second film will land in a comparable space — through a different lens.
What Sea Prince and the Fire Child is
Animated family films don't often reach for elemental allegory, but here we are. Two youngsters, scions of fire and water deities, discover each other and, naturally, fall in love. What follows is a fairly standard clash of clans, albeit with gods and cute animals.