If you loved Codename: Kids Next Door: Operation Z.E.R.O., try The Grim Adventures of the Kids Next Door
A bridge between a film you've already seen and one most people haven't. The Grim Adventures of the Kids Next Door has roughly 4.2× fewer votes than Codename: Kids Next Door: Operation Z.E.R.O. — 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.

Codename: Kids Next Door: Operation Z.E.R.O.

The Grim Adventures of the Kids Next Door
What they share
Both films are directed by Tom Warburton, and they both carry the playful mood tag, and they sit in Action / Adventure / Animation / Comedy / Family / Fantasy / Science Fiction / TV Movie territory. If that's the register that drew you to Codename: Kids Next Door: Operation Z.E.R.O., the second film will land in a comparable space — through a different lens.
What The Grim Adventures of the Kids Next Door is
Suburban cul-de-sac. Perpetual afternoon. Lawn sprinklers hissing. A certain spectral scythe, some unlucky trousers, a plea for help. Kids Next Door answer the call, only to face the wrath of Mandy, the Delightful Children, and a ghastly fusion neither side expects. Cartoon Network mashups rarely felt this demented.