I'm gonna say it: Yuuzhan Vong!
Sorry to crush your dreams here, but the Yuuzhan Vong are absent from the Force, so she wouldn't be sensing them. Secondly, the Yuuzhan Vong don't invade the galaxy until thousands of years later. Putting them into Swtor would only take away from the "Invading Aliens" vibe. They were almost unknown before the invasion and if the Sith Empire and Galactic Republic joined forces to eliminate them, they would be well-known to the general public as the invaders that brought the Empire and Republic together and ended the Great War.