Recent patches to the Public Test Realm have revealed some possible outcomes of Battle for Azeroth's story. None of this can be said for certain, but Blizzard has been known to love foreshadowing. What we can do is give educated guesses.
As the release of World of Warcraft: Battle for Azeroth approaches launch day, some story content is slowly being put on the Public Test Realm. Of course Blizzard won't reveal everything even though they no doubt have this and the next expansion basically planned out.
What has recently been released is the attack on the town of Astranaar in Ashenvale. Seeing as the Night Elves have left their cities and towns undefended to go to Silithus and mine Azerite for their own purposes. The whole ordeal begins when the players are tasked to travel there and see what happened, only to find the entire population dead by the hands of Sylvanas' Horde.
What this will slowly culminate into is the burning of Teldrassil, the Night Elves' most sacred tree and essentially their home. We saw this scene in one of the first videos detailing the story of Battle for Azeroth a bit further. The reason behind all the power struggles between the Alliance and Horde was thought to be because of the Old Gods who are pitting them against each other, but this is still unconfirmed.
Next up, the appearance of a certain woman in the game is sure to make quite a stir in the later stages of the game. She is Bolvar Fordragon's daughter, who took the mantle of Lich King after Arthas was defeated. What this might mean is that Blizzard is slowly setting up Bolvar's awakening from his four year slumber. Whether this will happen in this expansion or the next one, we cannot say since we do not really know what his motives and capabilities are.
The last addition to the game's most recent version is a changed appearance of Grommash Hellscream. In Warlords of Draenor he sported an upright stance and black hair, but now his appearance has changed to that of an older Orc. Why this is so we cannot tell since we have no reference to how much time has passed since Warlords of Draenor.