I just realized a horrible truth:
Darth Nihilus and Darth Sion have been ripped off to ruin two other unique characters--Vitiate (The Emperor) and Revan.
My reasoning?
Vitiate > Nihilus: Well, let's see. He can transcend the physical realm, he wants to eat the galaxy, and he created a device called the "Ravager" in the Sith Warrior story.
Revan > Darth Sion: To make my point undeniable, I'm going to reference Revan's (gag) light side half: "You've carried on, dragging the remains of a body that should have long since faded to dust. Hatred fueled cunning, but burned out all wisdom. Without me, you could not see."
Sigh.