The best magic items don't just add numbers. They create moments.
The 3-Layer System
Layer 1: Mechanical - What does it do? Layer 2: Narrative - What's its history? Layer 3: Dramatic - What tension does it create?
Example: The Lantern of Lost Voices
Mechanical: Once per long rest, speak with a dead creature. They answer one question truthfully. Narrative: Forged by a grief-stricken artificer after the Siege of Ashenmere. Dramatic: After 10 uses, the whispers begin answering questions nobody asked.
The Golden Rule
Every item should create a scene. If it only adds +1, it's boring.