As one could predict, the alleged details of the upcoming Omega DLC for Mass Effect 3 have been leaked. Keep in mind that these details are leaked material and may be incorrect since I can't find the original source for the leak, but they sound reasonable enough:
“Aria tells you that Omega is now under a blockade from Cerberus. Since a direct attack didn't work, Cerberus is now trying to starve Omega into submission by blockading supply ships coming in through the regular Omega relay. The level's appearance will change to reflect this - the Afterlife has gone quiet, the music is off, the flashing lights are gone, maybe a soup kitchen has taken over the dance floor. Aria requests that you steal into the blockade and board the command ship. Take down the blockade so Omega can be resupplied.
As punishment for breaking the neutrality pact, Cerberus launches an attack against Omega. The enemy is attempting to destroy Omega's shields, which will allow asteroids and meteors to smash into the station. You must repel the assault and ensure the shield remain operational.”
One glaring inconsistency is that the leak also mentions that Zaeed Massani is involved. Despite the events of Citadel: Volus Ambassador in ME3, Zaeed is apparently working for Cerberus in the DLC and Shepard may have to kill him. I can't figure out how this would work story-wise unless the Omega DLC completely supplants the Volus Ambassador mission, which would make the DLC difficult to reconcile with the story if players have already completed that mission.
Notably, Zaeed working for Cerberus was also a plot point listed in the original ME3 leak from last year. Shepard was to have found Zaeed in the wreckage of a Cerberus ship and learn that he had been hired by Cerberus to protect a high-ranking officer; Shepard had to convince Zaeed that Cerberus was up to no good or be forced to kill him.