I'm telling you, at least Kul Tiras, Zandalar and Zin azshari are all going to be packed in one expansion. It doesn't make sense to split them as there wouldn't be enough left to make another expansion in those areas. Especially if it's going to be naga focused.
Some parts will probably be available in 8.0 and then they'll probably launch the other locations as we go. Much like how they did it in MoP.
Just check Argus, it's a huge area split into three parts. They also made Mardum which people seem to forget is a whole zone in itself.
I don't think we'll see Kezan and the Undermine, I feel like they closed that chapter when Deathwing nuked the area in Cataclysm.
There's also Ny'alotha but I think that'll be used as the bridge between 8.0 and 9.0
I think some guy in MOP hinted towards Zandalar being destroyed but we don't know that for sure. But I don't think people have to automatically think that they'll pull a Broken Isles on us, for all you know they'll make it so you have to travel between the zones using ships and sea mounts.
Had they made Kul Tiras and the surrounding areas into one land mass, I'm pretty sure people would whine as it'd be the same recipe they've used for far too long now.
And then there's the question about a possible world "renovation", we already know they want to apply level scaling to the old zones (probably 1-60 in vanilla zones, 60-70 in tbc, etc) and that'll probably mean they want to fix or at least redo some parts of current Azeroth, maybe even Northrend with the current updates to undead creatures as well as the Chromie stuff.