EA has confirmed during their Summer Showcase that Mass Effect 3 will be getting some single-player DLC, and it's the previously rumored Leviathan add-on.
This is the official description from EA and BioWare:
Mass Effect 3: Leviathan is the first story-driven, single player DLC to be made available since Mass Effect 3 Extended Cut. Leviathan expands upon the events of Mass Effect 3 with gripping and emotional storytelling, compelling new characters, powerful weapons and unique upgrades. Players will be able to discover more about the origins of the reapers as they race across the galaxy to unravel the mystery that surrounds the fabled Leviathan.
Pretty vague, not surprisingly, but the add-on should delve deeper into the Reapers' origins. The description actually doesn't tell us nearly as much as we already know. When rumors of the Leviathan DLC first surfaced, the leaked info already hinted at the nature of the Leviathan.
Spoilers begin here. Based on what has been uncovered, the Leviathan appears to be a Reaper who was exiled for killing one of its own. Fascinating, no? The DLC events are supposed to occur around the same time as the Mass Effect 3 ending, while Shepard is having his drug trip of an adventure, meeting the Star Child, and all that jazz. Spoilers end.
During this year's Comic-Con in San Diego, BioWare did show a still image of a mech, presumably to tease the Leviathan DLC. We couldn't tell you for certain what else was in the image, however, because we were about 50 rows away from the angled projection screen.