If you know how video game designers enjoy putting hints in their games to give away what is to expect next, then this shouldn't be a surprise to you.
After playing through Bioshock, I noticed right before you confront Andrew Ryan, in a sub-room, Ryan has a jumbled wall of pictures and news clippings with the words "Would you kindly?". The main part of the wall has multiple pictures with strings to show the connection between one another. There is a picture of a shadow with a question mark. As seen below.
When I saw this I wondered to myself, everyone is somehow tied to each other in one way or another. But this shadowy figure is the only one that is tied to Ryan, and cut off by Ryan. so I researched into all the audio diaries. The only diary that has someone not within the story line is a man named Rosenberg. By the sounds of the audio diary, this Rosenberg character is a superior of Mr. Ryan.
"Transcript: Diane, my dear, I'm sorry but I'll be late again tonight... Rosenberg is demanding to speak about this Fontaine business. I'm trying to build a proper financial market and this idiot keeps going on about Adam this and genetic modification that. I'll go spend an hour pretending to pay attention to the poor fellow and be home as soon as I can."
Rosenberg is the most likely new character for bioshock 2, for the simple fact that rumors circulating the Bioshock 2 community state it will be both a sequel and prequel, that fits perfectly for Rosenberg. He was there before and he is still there.
