BioWare recently released about thirty seconds of teaser footage for the new Mass Effect game, which had distorted dialogue but did show what appears to be the construction of a Mass Relay, and one fan has managed to crack the dialogue. It suggests that whatever humanity is doing, it does so in defiance of the new Mass Effect game's council.
How exactly a reconstruction of a Mass Relay (which could also theoretically be a Super Mass Relay to connect the Milky Way and Andromeda galaxies or something more sinister) is done so in defiance is unknown. It doesn't bode well that the reconstruction seems to be led by Cerberus. It's possible that this technology is being built with leftover Reaper tech, given Mass Effect 3's Destroy ending, and that would make sense. Control and/or use of these resources would likely be heavily regulated.
Either way, BioWare's Michael Gamble credits Twitter user @eughenia_sh for decoding the transmission, as heard below. It seems to be taking place between an unknown alien race not speaking common (or via another transmission) and Liara. The ending is definitely foreboding for the next game, as Liara T'Soni speaking of the council states that "they should know by now not to underestimate human defiance!" Gamble also retweeted this line, hinting at Liara's titles of Doctor, Shadow Broker, and "<REDACTED>." Liara has played a big role in Mass Effect 4's marketing so far and will likely continue to do so.
Of course, such a teaser opens the door to tinfoil hat territory, and a lot of the language here could hint at something more. First off, the official Mass Effect Twitter thanks its "Agents" for decoding the transmission, suggesting that whatever faction the new player begins as part of, it's likely whatever features these "Agents." Now, the construction is being recorded by another ship with a Quarian Captain, likely meaning that these Agents are watching over humanity, if not more of the galaxy's folks, somehow following the destruction of the Mass Relays. Liara may somehow be part of that, hence the redacted title.
Players will likely begin the new game as one of these Agents, joining Liara in some form of a quest. Now, BioWare is doing everything it can to paint the construction of this Mass Relay in a bad light (Cerberus' involvement, Liara's dialogue) with this Mass Effect 4 footage, but it'll be impressive to see if this is a red herring. A fragmented Milky Way Galaxy potentially controlled by a fearful, powerful new council would be a nice direction to encourage defiance; it's not likely to be the title, but Mass Effect: Defiance has a nice ring to it.
The new Mass Effect game is in development.