If you loved A Certain Magical Index: The Miracle of Endymion, try Trinity Seven: Heaven's Library & Crimson Lord
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.

A Certain Magical Index: The Miracle of Endymion

Trinity Seven: Heaven's Library & Crimson Lord
What they share
Both films are directed by Hiroshi Nishikiori, and they both carry the playful mood tag, and they sit in Action / Animation / Comedy / Fantasy territory. If that's the register that drew you to A Certain Magical Index: The Miracle of Endymion, the second film will land in a comparable space — through a different lens.
What Trinity Seven: Heaven's Library & Crimson Lord is
A schoolboy inherits a cosmic library and a list of questionable allies. After his cousin abruptly vanishes into interdimensional voids, a demon-teacher offers three ways to patch reality, none of them safe. The real mystery may be whether anyone’s actually reading the terms and conditions.