"Until we get true AI which can alter the story instantly and in a satisfying way to compensate for anything the player wants to do, we have to compromise."
Eh? This is getting repetive. PST, and BG2 are two games prove that you can have npcs important to the story and game while still making them fully killable. Why do people keep talking about needing to compromise?
As for an expansion continuing the OC; I doubt it unless they add epic levels which is a possibility. Afterall, the OC in NWN2 will get you to level 16-20 most likely. In a Mr. Sawyer example, his character got to level 19. And, I doubt an expansion would only give you one level. LOL :D