One of the many annoyances in Legion was that the story was incredibly inconsistent and didn't feel like it built up any of the content patches that we had.
So BfA is essentially just a hard reset to the story, it'll delve deeper into each faction leader and prominent lore characters we've had around us but the main plot will be Horde vs Alliance.
Azshara makes an appearance, we do not know if we kill her or not, I surely hope not as that'd be a waste of a good villain.
But I have my doubts we'll see N'zoth until right before 9.0, he is not awakened yet, nor is his prison open yet, so this this is likely going to be the setup to an old god expansion in 9.0.
It's clear to me that they wanted there to be room to breathe between the Legion and the void stuff. A faction war is the perfect way to let that happen with small elements leading towards N'zoth. There's always been a fan theory that we'd fight Azshara but she'd defeat us, and then we'd kill her the next expansion.
And if anything, they've been really big on fan service lately.
Could be that they're setting up C'thun as a possible end boss though, with the blood that's essentially seeping into the ground in Silithus. Like in WoD, they had Archimonde suddenly become the final boss, C'thun might as well be that for BfA. It'll basically end where the plot for 8.0 started.
And then comes the invasion once C'thun dies.
I don't think N'zoth is going to make an appearance in this expansion at all, would also be weird to essentially have the same plot twice in an expansion (first the blood god and then N'zoth). So I'd say that solidifies that N'zoth is not gonna be here in 8.0