Marvel's The Defenders: 23 Easter Eggs You Probably Missed
3. The Hellboy Link
This isn't so much an Easter Egg as an unconscious narrative theft, if you're perfectly honest with yourself.
It was always going to be a brave move to focus so heavily on Iron Fist after his show was the least well-received, but the show did pretty well to make him appear better than he did in his own series. They achieve that my stealing the plot of Hellboy and repurposing it under a thin vein.
In both cases the hero's arc is based on him realising his true, dark destiny, being used as the key to open a mystical door and unleash doom on the world in the process. In both cases, the hero's hand is the literal key to said door. All they needed was to have Danny Rand grow horns and it would have been the same film.