glint in (one's) eye(s)
1. A brief and subtle expression in one's eye(s) indicating a hidden or secret emotion, agenda, idea, plan, etc. She said that she was happy for both of them, but I knew by the glint in her eye that she despised Mark's new girlfriend. My father, with a glint in his eyes, told me that I would get the best present after the party was over.
2. A person or thing that has been planned or discussed but has not yet come into being. Jonathan says he's going to write the great American novel, but it's just a glint in his eye at this point. I knew your parents back when you were just a glint in their eyes.