I do maintain that Sylvanas wasn't irreplaceable—she had a pretty clear character from Warcraft III - Cataclysm before she showed the first of her inexplicable brain damage coming in with War Crimes and fully committed to her self-inflicted lobotomy during Legion, I agree, but I don't think she was wholly irreplaceable. I've mentioned Faranell ad nauseum already, but I really do think he's one of the best "traditional" Forsaken in the game—even if there aren't really many. He was high up in the echelons of the Forsaken and always struck me as more pragmatic than his cartoonishly evil counterparts in the RAS whilst still having that sinister edge that suited the Forsaken.
I can't really bring up many more, which I think is actually reflective of the issue—the Forsaken never got an opportunity to develop their Cataclysm identity much more. They had a good start with Sylvanas' Lordaeron Anschluss, but we never really saw many of the Cataclysm Forsaken in the spotlight after their questing experience, which gave them very little wiggle room to develop. By Legion, they were sort of thrust back into their old role (no matter how nonsensical it was) and were now just orbiting Sylvanas' character. Ultimately, I could see a salvaging of the Forsaken post-Sylvanas if there were a coup by hardliners in Sylvanas' military. There would be a De-Sylvanasification akin to Destalinization post-Stalin, but it would ultimately remain under people hardly better than Sylvanas whilst still giving them good reason to be kept around in the Horde without anyone batting an eye because they've given the illusion of reform.
I think one interesting plot that could've come up with Sylvanas in the spotlight is a July 20 Plot parody, with the most pragmatic Forsaken, realizing that Sylvanas has finally gone off the deep end, plot to kill her off. This isn't actually out of any sincere moral disgust for all but a few legitimate objectors, but is instead a desperate attempt to save their own decayed skins. Belmont and Faranell could've been among those who were involved, and it would ultimately culminate in the slaughter of most members save for a few, who manage to make enough of an impression on the Horde that they assume they are actually the harmless, well-meaning revolutionaries they present themselves as. To add to their appearance of legitimacy, they would try to prop up the Desolate Council's remaining members, but in actuality would only be using them as mopey puppet leaders while the inner circle puppeteers them from behind the scenes.