Since metal is (usually) silver in color, scratches should't be hard since you just make a small, repeatable texture that move in a direction (ex: right to left scratch), when they touch an object going greater than a certain speed it creates a scratch in the direction they are moving with various lengths (ex: pickup scraping a wall at 60 mph creates a long rough scratch right to left).
i once made a list of all suggestions long time ago. And yes it was suggested before but still waiting