1. You can't kill Demons, because as you see at Archimonde, you have to slain them inside the nether to kill them. Which means, all those Demons we're killing right now, doesn't matter, because they are infact not dead. This also means, we can't stop the Legion. We can only fight it back. This also means, there would be an impact of the Legion in WoW foreever, because it will never be gone completly. This means, if we want to get rid of the Legion, there is only one guy. Sargeras.
2. In the last phase of the scenario, right infront of Gul'dan, the area is called "Tomb of Sargeras".
3. If you run straight towards it you die. If you check your combat log, it will say: "Vision of Sargeras" 300yard range. This is basically a ability of sargeras, which also means, he is somewhere there. My guess is, he is inside this Tomb of Sargeras. (orly??)
4. Moreover, if you approch Gul'dan, there is a message (translated into english) "For one moment, you feel an overwhelming presence behind the portal." This could be the Legion itself, or Sargeras. But since they are talking about an "overwhelming presence", it's more likely something bigger than just some random named demons.
5. We have to be serious. There was the Iron Horde, so the Endboss had to be Garrosh. Now there is the Legion, so the Endboss can't be corrupted Jaina oder Demon-Varian or Super-Mega-Death-Hogger. It has to be a boss which is completly related to Legion and can introduce the new expansion after Legion. Let's say we are intented to lose against the Legion. The next Addon after Legion, would have to be another Legion addon, because since they won, they destroyed Azeroth and everything is now green and demonish etc. But if we would really lose against the Legion, this will be a huge plottwist in this expansion. Blizzard would make us believe, that we would never lose. We would kill every demon and in the final fight, we would be looking as the winners, but instead of actually winning, there will be that cinematic or moment, which proves, that we lost everything, and therefore, everybody can go "WOOOOAAAA HYPE". So loosing against the Legion, would result in leaving Azeroth, which is also the perfect opportunity to create a complete new world, maybe even planet, as the new main stage for our characters.
6. Winning against the Legion, would have to be a true victory, which is only possible, if the kill Sargeras.
7. Killing Sargeras isn't really possible in the general case. Because it's fucking Sargeras. He kills us while looking at us. But, we also know, that Artefactweapons will be useless after Legion and also are one of the main features in Legion. So on the one hand, there is this super strong villain, that has to be killed and on the other hand, there are those super strong weapons, which are intended to be trash after legion. This is basically 1+1. We use our weapons, altogether and smash his demon soul, which also kills him and corrupts our artefacts and make them useless. (after fighting him for 30mins ofc)
8. Im not sure, if we're intended to loose against the legion or to win, but I'm sure, that last boss has to be sargeras
9. If Sargeras is really there (in the scenario), you have to ask yourself: Why is he there, why isn't he showing up and on the other side: What is a Legion without his leader? Because if he is there, like physically in the game, than we have to deal with him from one time to another.