Let's look a bit at the war between the Alliance and the Horde.
Vanilla: Tensions. Peace is breaking. Big world to explore and battle between the Horde and the Alliance in some zones. Temporary truce to fight the Qiraji.
TBC: More Tensions. Still a big joining of forces, but battles here and there.
Wrath: Varian returns. Conflict between Varian and Garrosh. Forsaken attack both the Alliance and the Horde. Sylvanas is distrusted. Horde and Alliance hatred at a maximum. Stab each other in the back. Even fighting in Icecrown Citadel. Somehow manage to kill the Lich King even though not really working together.
Cataclysm: Garrosh rises to power. Warmongering. Horde mostly on the offensive, Alliance on the defensive. Fights all over Azeroth as the world is breaking apart. Beginning of a new kind of hatred that culminates in the destruction of Theramore through a manabomb. Jaina, a peace-loving woman is consumed by hatred.
MoP: Alliance and Horde bring their war to the shores of Pandaria. Cause the eruption of Sha, that were sealed away, through their emotions. Blood Elves kicked out of Dalaran after betrayal. Garrosh rises to become hated by Horde and Alliance alike. Both work together to dispose of him. Still tensions after the battle at Orgrimmar is over.
WoD: Alliance and Horde wanna focus on the Iron Horde, but sometimes still get in each other's way. They just can't let it be. Jaina certainly still wants revenge.
Legion: The Legion returns. Varian sacrifices himself at the Broken Shore, when the Horde abandons the battle, because Sylvanas ordered the retreat. The Alliance wants revenge. Genn goes after Sylvanas in Stormheim to stop her and to get revenge for Liam, Varian and Gilneas. Succeeds. Tensions between Horde and Alliance very high again. People on both sides wanna go for it, but the battle against the Legion is too hard, and too important, and everyone needs to work together for us to defeat them. It's not a battle we can win while fighting each other on the side, like in Icecrown Citadel. We have to work together and lay our petty differences aside. For now.
After Legion: Sargeras is banished. The Legion is stopped (for now). Of course they will return eventually. But for now, it is the best time for the Horde and the Alliance to fight each other. Everybody expects that a new enemy will appear. Like the Void or the Naga. The hints are there, but there is nothing that points to it happening immediately after we defeat Argus and seal away Sargeras to stop the Legion.
So what are people like Genn and Sylvanas gonna do, once the big battle is over, and there is probably the biggest peace we've ever had since the Battle for Mount Hyjal? Go to war.
Sylvanas is gonna try and defeat the Alliance. Genn, Jaina and maybe even Anduin, are gonna call for war against the Horde to finally defeat them and to get revenge for Varian. If there is no other force invading at the beginning of the expansion, then the main force might be the other faction this time. It's at least the perfect time for it to happen. Then maybe, during the middle of the expansion, we accidentally set free some ancient power. Maybe Alleria fights on the side of the Alliance and her messing with the Void leads to us summoning the Void, which will be the surprise end raid of the expansion.