Just to say, the SI can't be pureblood. Because he was a slave and in no possible canon scenario could a sith pureblood become a slave, especially if force sensitive, as they are an elite race. So if it ever is canonized, he or she will be human (he he he, by SI is human
And seeing as Ravage has entered the discussion, I'd like to see him as the next opponent for the SI (not for everyone, just the SI storyline). Like Thanaton he seems a bit backward in his ways, and is opposed to you taking a seat on the council, so there's going to be some rivalry their. Also being the head of Expansion and Diplomacy, he's going to play a bigger role now the Great Galatic War has restarted.
Karrid is also an interesting prospect. Notice that their have been no Faleen NPC's in SWTOR - so this would be interesting.