Basically, my idea is that what Faction you choose at the end of "The Nathema Conspiracy" will gain you access to either a Level 70 post KOTET version of Dromund Kaas (if you chose to ally with the Empire) or a Level 70 post KOTET version of Coruscant (if you chose to ally with the Republic). Also, you will meet certain characters from your class story, for example, as an Imperial Agent, if you spared Ardun Kothe, he will greet you when you first land on Coruscant
The two areas will share achievements (for example, an achievement example would be "Kill X Non-Player Enemies in the Coruscant or Dromund Kaas Daily Area")