According to Mass Effect 3’s Lead Writer, Marc Walters, a Citadel plot line that was cut from Mass Effect 2 will appear in the game.
The Citadel was not nearly as prevalent in Mass Effect 2 as it was in the original; very seldom did a quest actually require you travel back there. When Official Xbox Magazine asked about the decision to include the plot into Mass Effect 2, Marc Walters stated:
“There was a Mass Effect 2 plot that was a kind of callback to the first Mass Effect that was going to be on the Citadel, and we cut it. But now it’s made a resurrection in Mass Effect 3, so I’m happy, but I can’t say what it is. That’s the nice thing about trilogies, sometimes you get a second chance.”
Personally, I did not care for the citadel after the first Mass Effect game because it was so centered around the Citadel, I could go another Mass Effect game before I missed that place. Hopefully this just means that there is one additional reason to visit this place that will help further the story.