Pelagos might be motivated to release Sylvanas at some point when growing threats cannot be contained any longer. We are going to need those Dark Ranger players after all. Heroes (the players) going to need all the help they can get in order to fulfill their destiny. And there is no better master Dark Ranger than Sylvanas herself.
What's next for Sylvanas?! Rooting in dungeon doesn't do anyone any justice. We know why pentagon always call the rebel in any movie ever in order to help them eliminate the bad guys. To successfully fight the savage you actually need to hire a fucking savage. Sylvanas is a solution for that. Dark Rangers will become inevitable at some point, and Sylvanas will be the key to unlocking all secrets.