Not sure why people always desperately push the blood elves into "her faction".
The blood elves care about the best faction for them to continue their survival. A faction that is built on the premise that everyone needs to die and become the bitch queen's obedient rotten slaves is not exactly in that spirit. Also Lorthemar has only treated her favorabily because of her sacrifices back during the scourge days, in truth both of them have been visibly at odds since SoO and with the recent Baine affair he has already stated to the player that little of the noble spirit that sacrificed herself for her people remains.
Something similar is true for the goblins. Gallywix wants profits, there is no profits with the forsaken. There is profit with the living, with the horde, that means the tauren, trolls and orcs, maybe the belfs.
The only race that actually has precedent for mindless slaughter would be orcs and maybe some of the troll tribes and the grimtotem. Certainly not the darkspear though. And even the orcs are not really on board with her usual enslaving the dead. That is more along the lines of the shadow council which the horde orcs are not to fond of.
Also worgen joining her? Lol, they were by far the most opposed to her before she slaughtered half of the nelf race, now they are eclipsed only by them.
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Highly unlikely in the first place, but they could also shift the factions. Allow all horde races to join her, allow all alliance races to join the new Thrall/Saurfang Horde faction and keep the alliance itself pure to them. That way you would essentially create a new neutral faction and have a spectrum from evil over neutral to good.
The issue with the idea of extremist factions on both sides is that, at least with Sylvanas, it no longer makes sense. The premise would be more along the lines of racial extremism, that could work by having Tyrande and Gen lead the "fuck the horde" faction, but Sylvanas is already beyond just extremism of "fuck the alliance". Her goal has already been shown to beyond just that scope and Horde doesn't currently have any decently built characters that could lead such a faction. The closest would be maybe Talanji but that is.. eh still to early for her imho. The horde has fucked the alliance on so many occasions, not sure who among them would even have enough of a grudge to spear head the extremist faction without having a character drink the coolaid first. Is Romanoth still around? Maybe if we dig up all the characters that Jaina has wronged we have enough to outweigh one actually popular one.