About all we know is that it's called "Citadel" or at least seems focused on the Citadel. My guess is it will uncover some mysteries of the Citadel's original construction and purpose-- but of course not enough that the events at the end of the game would no longer be a surprise for Shepard. I don't know, Omega was an obvious tangent for a DLC, but no "unfinished business" relating to the Citadel springs to mind. Maybe something that will touch on the Armistan Banes mystery from the first game?
There's also the possibility that this will be the last piece of campaign DLC for Mass Effect 3. If that's true, and I don't know if there's much evidence one way or the other, then it's much more likely to be a "grand farewell" that has a lot of relevance for the main story and characters than a standalone side story like Omega.
Would be nice if did cover Armistan Banes, as it does appear that it might be an interesting lead-in for some side story:http://masseffect.wi.../Armistan_Banes
If it is the last piece of campaign DLC, they do need to do some big send off to wrap up some loose ends and tie things up neatly.
That is assuming we don't have Mass Effect 4: The Retcon.
Hoping it's more side story since I decided to sell my copy off after Omega--as after that I didn't figued I'd want to shell out for anymore DLC.
I'd lean toward it being that. Hard to do much more with the main story. At most it will flesh out some more backstory of the Citadel, but as Ryu notes it can be anything major that spoils the ending for people who play for the first time and do the DLC before beating the game.
I'm sure you could rent/borrow it somewhere along the way if you did want to go through that DLC at some point in the future.