Most of my writing about Sylvanas' role in Legion has been focused on her rivalry/being on the receiving end of one hell of vendetta with Genn Greymane. But while I think that the King of the Werewolves is going to certainly be a big factor in her plot (and hopefully he'll have some plot of his own that doesn't involve her,) there's another guy who we might want to keep an eye out for.
The Scourge was originally a tool of the Burning Legion. The Third War can accurately be called a war between the races of Azeroth and the Burning Legion, but the Legion was mostly using the Scourge as its proxy force - a far more effective one (even if it was ultimately unsuccessful) than the Horde.
The Horde was unwieldy, and while Gul'dan attempted to manipulate it from behind the scenes, his Shadow Council was unable to keep Doomhammer under control once Gul'dan fell into a coma (partially because his first act as Warchief was to kill nearly all of them.)
The Scourge was all directly subservient to the will of the Lich King, and the Legion attempted to control it by sending a number of Dreadlords as Minders to make sure that he stayed on task. Of course, the Lich King used Arthas to undercut them (there are a lot of parallels between Arthas and Kerrigan from Starcraft, especially after the things we've learned in Starcraft II, though Kerrigan, at least as of the end of Heart of the Swarm, still seems potentially redeemable.)
Anyway, after Illidan tried to shatter the Frozen Throne using the power found in the ruins of Dalaran, the Plaguelands erupted into a three-way Scourge Civil War. Those loyal to the Lich King fought with the newly-freed Forsaken while this team of dreadlords tried to take this opportunity to assume direct control of the Scourge.
The three dreadlords given this task? Mephistroph, Balnazzar, and Varimathras.
Balnazzar would possess Saiden Dathrohan - one of the first Knights of the Silver Hand - and form the Inquisiton-like Scarlet Crusade, turning humans in the area into paranoid religious fanatics. Mephistroth hasn't done much, but he'll be in possession of at least one artifact weapon in Legion.
And then there's Varimathras.
If you started playing in Wrath of the Lich King or earlier, you probably remember seeing a big demon guy hanging out in the Undercity. Varimathras was defeated by Sylvanas (it's actually the mission if Frozen Throne that I got stuck on before I sort of drifted away from the game,) but instead of allowing himself to be destroyed, he instead swore allegiance to the Banshee Queen. He served the Forsaken for years, even founding their spy agency, the Deathstalkers. It was an alliance of convenience, to be sure, but clearly Sylvanas never thought that his eventual betrayal would go quite as catastrophically as it did.
Secretly, Varimathras had won the loyalty of the Royal Apothecary Society, or at least Grand Apothecary Putress. As the Alliance and Horde launched their fight against the Scourge, Varimathras took his opportunity, launching a coup d'etat in Undercity and betraying both sides. Putress launched the plague he had been developing for Sylvanas on the Alliance, Horde, and Scourge alike, killing some of the finest troops in Azeroth. While this major event at the Wrath Gate was happening, those loyal to him attempted to kill Sylvanas.
The Banshee Queen escaped Undercity and fled to Orgrimmar, leading refugees as Thrall put his own city on martial law lockdown. Eventually, both factions marched upon Undercity - the Horde to defeat Varimathras and the Alliance to destroy Putress.
Varimathras was using the royal chamber to summon swarms of demons, answering calls from a "Dark Voice" that many have speculated is Sargeras himself. Ultimately, Varimathras was slain, but we all know now that as a demon, he was not permanently killed.
With Sylvanas rising to prominence in the Horde, Varimathras is a potentially devastating foe - someone who knows her well, and knows how she leads the Forsaken. Personally, I can't wait to see the confrontation between them. I'm sure Varimathras is excited for round two.