I would have been much more interested in a prequel happening before or during the fall of rapture to madness, would have been a greater opportunity for gameplay and storytelling IMO.
A couple of the previews hint at playable flashbacks. I was hoping for something like that, mainly as a form of extending the radio recordings. Instead of just listening to people talk, the player should be able to actually see those events in first person for some of the recordings.
Take it a step further and have one actually control the person whilst the character narrates. In the game, have goals for the player or something. Let the player take time to see Rapture as it was (within the area of interest) and you'll have that immediate contrast between what you see in 1967 versus the 50s. Let's have that moment of "Wow. What a change. How the mighty are fallen."