In an interview with with OXM, Mass Effect 3’s lead writer, Mac Walters, revealed that missions cut out of Mass Effect 2 that take place in the Citadel will be in Mass Effect 3. The Citadel was a fundamental location in the first game but not so much in the second. The Citadel in part 2 had restricted the freedom of exploration around the location and only a few missions took place there.
“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.”