The new DLC is simply titled “Omega.” From its title, we can safely assume that the DLC will revolve around Shepard helping Aria take back Omega from Cerberus’s control.
There is no confirm date or price for the DLC yet. We will have more information as it becomes available. Also just another tiny tidbit, a new Mass Effect game has also been announced.
Please note that it is very unlikely that Shepard will star in the next Mass Effect game. Bioware has stated numerous times that Shepard’s story will end with the trilogy. However, they also have expressed interest in exploring the Mass Effect universe further more, so expect to play the next game in the series with an entirely new protagonist.