Version 2.0. Trying to take in comments and suggestions regarding my first draft…
Have Tyrande become less and less herself. Most avatars of the Night Warrior would have died by now, as this is a power no mortal is supposed to possess for this long, but Tyrande endures. The Night Warrior persona is slowly taking increasingly greater control over her. It’s granting her even more powers, but at some point, it stops being in her control as a means to dispense justice and takes over with its goal of lashing out in pain and vengeance. At some point, Shandris or Malfurion try to speak to her, only for her to reply something along the lines of, “I am not Tyrande. I am not Elune. I am their pain. I am their suffering. I am their vengeance.”
Now under the near total control of the Night Warrior persona, Tyrande and the Army of the Dark Moon start to become an even greater force. Using Darkshore as their base, they spread out from there. They move into Ashenvale, Felwood, Moonglade, Stonetalon, and Winterspring, but they don’t just restrict attacks to military settlements. They also attack civilians, Horde Druids. Really play up any friendly/neutral Druids, Tauren, Vulpera, or Blood Elven deaths.
Meanwhile, the Horde council doesn’t want a full blown war, but they can’t just sit by and do nothing while Tyrande’s forces ravage their villages and kill indiscriminately. They send the hero to help evacuate people and bolster local defenses, but the Army of the Black Moon keeps advancing.
So the Horde leaders go to Anduin, Genn, Malfurion, Shandris, and other Alliance leaders for a solution. Genn doesn’t want to intervene, arguing that the Horde deserves this fate. The others, being the much more forgiving people they are, don’t wish to see every Horde man woman and child wiped out either. Shandris and Malfurion are primarily concerned with getting the real Tyrande back. They also explain how having the Night Warrior in mortal form means that only half of Elune is currently locked into a mortal entity- which means the rest of Elune is only at half power.
The best course of action is determined to be separating Tyrande from the Night Warrior. Tyrande gets to be herself again. Elune is made whole. The slaughter of civilians ceases.
So begins some quests to gather mcguffins, interspersed with some skirmishes with agents of the Army of the Black Moon. Under the leadership of Shandris and Malfurion and alongside the Night Elf forces who are not part of the Army of the Black Moon, they’ll face the Night Warrior and her army.
It all ends with a big climactic battle in which Aduin is fatally injured, and after some big emotional beat or after she attains her “FINAL FORM” and we defeat her, Tyrande is able to break free of the Night Warrior’s influence for long enough to see what she’s done, and how the desire for vengeance at any cost has only resulted in more death and is on the verge of plunging the whole world back into war again.
Tyrande manages to excise the Night Warrior from herself and Elune is restored to her fullness. Anduin dies, and in Before the Storm he actually named Genn his successor. So Anduin gets to die a tragic death as a saint, and the new King of Stormwind is a guy who really doesn’t trust the Horde. Since Genn’s also not a blood heir, and the King of Gilneas, you could totally start another storyline inside the Alliance, where the people of Stormwind -especially the nobility- refuse to accept him, and he sees challenges to his throne and authority.
Meanwhile, the territory gained in the fighting remains under Night Elf control, with only the southern parts of these territories abutting Horde lands being contested. Genn holds firm on this, as he was always sympathetic, and doesn’t want to give the Horde anything.
And thus, we return to a cold war state.