"I can do this all day." Marvel certainly aren't shy of including Easter eggs and references in their movies, but it was still a nice surprise to hear this line coming from Cap, which is a direct callback to the same line uttered in Captain America: The First Avenger. There, it's when he's faced with a bully before being saved by Bucky; here, it's in a showdown with Iron Man. He's come a long way, but the line signifies that he still has that same heart and fight inside of him, and also probably ties into Bucky in some way (likely coming full circle, as he defends Bucky here).