Optional form fields are great but, if the user has used them, it would be helpful to be able to add some instructions before or after the optional input. Ideally then, there would be two inputs for an optional field where the instructions go (before or after the optional text) and the instructions.
Example:
Input - KEYWORDS
If it is blank, I don't want to include anything. If it is complete I might want to add
You must include the following keywords: {KEYWORDS}
So now I would be able to add "You must include the following keywords: " to my field. If the user doesn't fill this in, nothing is added to the instructions. If they do then the {KEYWORDS} placeholder is replaced with my instructions AND the user input.