We already know this DLC is going to be based around the Fallen, just like The Dark Below was based around the Hive. This has been basic public knowledge for a long time, although never really verified by Bungie. However, if you open your copy of Bungie you will find a flyer advertising the expansion pass - if you then look closer at the first, you can see it shows the face of a thrall while the second is the face of a fallen vandal.
As soon as more information is available for this DLC, you can be sure to find it here so don't forget to check back!