The title pretty much sums up what this post will be about, and although it is probably mostly personal opinion I actually think I have a decent enough point to make this a thread. Sargeras himself must die this expansion.
So by now you're probably wondering what the hell I'm going on about so let me explain. Sargeras is the leader and founder of the Burning Legion, so whilst he is alive the Legion will always be an existing and mighty force. As you might expect, the Burning Legion is mostly comprised of Demons who cannot be truly "killed" because when they die on Azeroth, they are simply sent to the Twisting Nether to regain their strength so that they can return once more.
Lets just quickly rush straight into the middle of my point: The Legion is invincible whilst Sargeras lives. If we defeat this Legion invasion, they will only return again which means another expansion based on the Legion invading Azeroth. Azeroth cannot be destroyed by the Legion because it would literally ruin the story progression and immersion of the game even more so than what the Cataclysm did.
The Burning Crusade had a lot to do with the Legion.
Warlords of Draenor had a lot to do with the Legion.
Legion is pretty much all to do with the Legion.
The only way to defeat the Legion is by defeating Sargeras. If we do not defeat him and only push him back, it will force another Legion based expansion in the near future. The next expansion literally cannot be anything to do with the Legion, because by then everyone will be absolutely sick of Demons.
Each expansion has an ultimate plan, a defining point that makes the story feel like it was relevant. In Warlords of Draenor, we setup the return of the Legion. In Legion, there needs to be something that happens to create the importance of the next expansion. Defeating Sargeras is the key, but only by killing him can we truly defeat him.
Disclaimer: I don't actually think we will kill, defeat nor banish Sargeras. I simply think its something that absolutely needs to happen to further progress the story of WoW.