I see this as conformation of a burning legion expac and not south seas.... although since azshara loves sargeras she might pop up.
The only thing that worries me is that the vision takes place in Undercity as opposed to Orgrimmar.
Orgrimmar is usually synonymous for the capital city of the horde & polar opposite of the human capital. I do think it foreshadows something, but I doubt it relates to any invasion on the specific capitals.
Next expansion is guldam traveling to the main timeline, going to the seas for the tomb of sargeras, and in the process we get the "great sea" expansion