A concept is not a vibe. Three parts carry it, and if one is missing the trailer dies on screen.

1. A named character. Not "a woman" — Detective Mara Voss. A name forces a point of view.
2. A forced change. Something rips the character out of normal life and they cannot go back. No change, no story.
3. Real stakes. What is lost if they fail — and we have to believe it.

Counter-example. "A moody film about loneliness in a big city, with rain and neon." That is an aesthetic, not a concept — nobody wants anything, nothing changes, nothing is at stake. Pretty, and dead.