An easter egg should be something that feels rewarding when you find it, so it should be well hidden and it should be almost a rumor. I would never fully reveal an easter egg, I would leave cryptic hints every now and then, and people can put two and two together. Like Shawn said, easter eggs can be anything you want it to be. From a cheeky reference to an unused music track, anything can be an easter egg. Just my opinion.