SPOILERS (if you haven't played the first part of bfa yet)
So I started to today and saw the first few cut scenes and I think I know how it's going to turn out.
Sylvanas needs to be killed. But Saurfang dies in the act, as well. That young troll who talked Saurfang out of surrendering to the Alliance at Lordaeron is adopted by Saurfang as a kind of son (because young troll's father died fighting with Saurfang -- and Saurfang's son died, so they go together). The Horde emblem (that Saurfang's son wore) that was returned to Saurfang by the young troll gets passed back to the young troll when Saurfang dies.
The young troll (Saurfang's adopted son) becomes the next warchief and represents the new honorable spirit of the Horde.
Any other theories?
Edit: More speculation -- Nathanos Blightcaller is too good of a villain to let go, so he will survive and be some kind of nemesis. Blightcaller probably will betray Sylvanas who is blinded by hubris (usually what happens to villains) and doesn't see the real threat until it's too late (like Sauron in LOTR, etc.). I think Sylvanas turn to evil might have something to do with Sargeras. Not sure what's up with Illidan Stormrage in all this. I don't think he's finished here.