If you loved Phineas and Ferb The Movie: Across the 2nd Dimension, try Phineas and Ferb: Mission Marvel
A bridge between a film you've already seen and one most people haven't. Phineas and Ferb: Mission Marvel has roughly 3.1× fewer votes than Phineas and Ferb The Movie: Across the 2nd Dimension — 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.

Phineas and Ferb The Movie: Across the 2nd Dimension

Phineas and Ferb: Mission Marvel
What they share
Both films are directed by Robert Hughes, and they both carry the cozy, late night, neon soaked mood tags, and they sit in Action / Adventure / Animation / Comedy / Family / Science Fiction / TV Movie territory. If that's the register that drew you to Phineas and Ferb The Movie: Across the 2nd Dimension, the second film will land in a comparable space — through a different lens.
What Phineas and Ferb: Mission Marvel is
Phineas and Ferb join forces with Avengers. They stop Dr. Doofenshmirtz. It gets predictably chaotic.