Fans have uncovered a bunch of dialogue for what appears to be a future Mass Effect 3 story DLC release hidden away in the code for BioWare's newly released Extended Cut DLC, which expands on the game's ending with the goal of answering questions that the stock ending left untouched. The script was discovered by the BioWare Social Network and shared in full on Pastebin (via Eurogamer). Take care before reading any further; there be spoilers ahead.
The script's partial dialogue for what most believe to be future Mass Effect 3 DLC nods to a rogue Reaper named Leviathan who has set up shop at a remote mining facility, indoctrinating those who live there. Shepard and his/her people discover what's going on with the colony when they're sent off to rescue a scientist, Dr. Ann Brynson. As events unfold in the Leviathan story, Dr. Brynson can either live or die and players can recruit Leviathan for the coming struggle against its fellow Reapers.
A peek at the Mass Effect Wikia reveals that Leviathan has actually been mentioned previously in the Mass Effect universe. It first came up in the original game, with players that scanned a certain planet discovering the remains of what appeared to be a living starship, the so-called Leviathan of Dis.
An even more specific mention comes up in Mass Effect 3, but only if the character Balak survived the events of the Bring Down the Sky DLC in the first game. Upon meeting Balak in ME3, you learn that Leviathan wasn't a Reaper corpse but rather an inactive one; scientists accidentially switched it on -- I wonder if Ann Brynson or her father (mentioned in the script) had a hand in this? -- and it proceeded to indoctrinate the entire system.
Whether or not any of this is true remains to be seen. The dialogue in the script definitely isn't in vanilla Mass Effect 3, and it alludes to events and characters that we've not yet met. Given that all of this is buried inside of code for the Extended Cut DLC, I'd say it's pretty likely that Internet sleuths may be onto something here.