Here's a scenario on how the War of Thorns plays out: Malfurion delays the Horde long enough for Anduin's reinforcements to arrive. Sylvannas realizes her gambit of capturing Teldrassil has failed. She orders the Champion of the Horde (the player character) to burn the World Tree in order to deny the Alliance their major port.
Would your character complete this quest? Would you do it if the quest offered a mount or pet as a reward?
Personally, this is how I hope the War of Thorns plays out. No Old Gods, no third party, no NPCs doing bad things behind the players' backs. Just place the responsibility squarely on the players.