I had the idea for this a few days ago but I couldn't think of how to embellish it. I think it's a strong enough icon to really stand on its own. Placed upon a banana, this sticker gives the idea that there is something great and mysterious waiting inside (and there is).
Our symbol system is really complex and crazy to think about. We unknowingly store data in our brains that allows us to unearth experiences and memories simply by looking at these symbols. The keyhole is no different. Current locks don't look like this at all, but still we associate the shape with the unknown and the untouchable.
However, the symbol system can get us into trouble when we really have to look at the world for what it is. One hurdle beginning artists have to overcome is setting aside what they "know" to truly see. Betty Edwards has tons more in-depth research and knowledge on this phenomenon in her great books: Drawing on the Right Side of the Brain and Drawing on the Artist Within. Check them out sometime.