depends on what flavor you want the card to have.
If you want a random creature at low cost, better to have fate egg to cost 1 less.
If you want a random creature at medium cost, but in a reliable way, let it hatch instantly, or being somewhat invulnerable when unhatched.
I like more the idea of a tiny, frail and defenseless egg, that hatches in something really poweful like a dragon or a nymph.