Well, I have been reading the last few pages and have been intersted in the "it can't be a new character/it can't be an old character" debate, and I must agree with the "it can't be an old/existing character" side. The great thing about Mass Effect is that it is in a galaxy with a huge number of people. It seems completely rediculous if that you, Commander Shepard, happen to recruit somebody who is related to the Shadow Broker (unless it is a public figure) in a Galaxy containing billions of people/aliens, it is just way too coincidential. In games like Bioshock (as an example) this is fine because the twist in that was already hinted at but more so because the game was "closed". By this I mean that there was only a few people you meet, the city was completely enclosed and the lesser people in the city would have been killed by the time you got there (so it makes sense that you'd only run into the important ones).
The big problem with making the Shadow Broker someone we don't know is that it lacks shock value (the creators of Heavy Rain new this and completely ruined the game by trying to avoid this situation). However, if the lead up to it is done right it can work. The only way to do this is to gradually find information about him and then, at the very end of the quest just before finding his identity, make the person either connected to an organization we know (this would be the best way in my opinion) or to a person we know (but not a relative as not only is that far too clichéd but it doesn't make sense, maybe an aquaintance or an old collegue).
Hoepfully I explained that well, and if Biioware do choose an existing character as the Shadow Broker I hope it is either a Geth (ok, not really an existing character but it makes sense as they share all information together in order to understand it), a member of the councillor (*cough* the Turian Councillor xD) or somebody in charge of an embassy (Shaira, for example, would be a good fit [although I cannot remember if she is the Asari in charge of their embassy, I think she is]).