Wasn't Ner'zhul supposed to have been "killed off" after Arthas became the dominant personality?
As for Kel'thuzad, he's certainly in a position where Blizzard COULD bring him back if they wanted to, but it's entirely up to them if there's a place in the story for his return. He's probably my favorite Warcraft character, but after how WOTLK handled the scourge, (Both Anub, Arthas, Ner'zhul and Kel), I'm tempted to say I'd prefer him to stay in limbo rather than be given a crappy ending. I'd rather not see a decent character turned to crap by mediocre writing like the "There must always be a lich king" nonsense.
I'd have loved to have never had Bolvar become the "new" LK and instead had Kel'thuzad come back with a vengeance some day as the scourge figurehead, he was left in control of the Eastern Kingdoms at one point and he was incredibly loyal to Arthas, so he had the credentials.