You are severely understating the content in Midgar. It's collected areas equal about half of the other places you listed at least and they can potentially add some of the other sectors, and the story content within it is pretty fucking stacked. I can see it being it's own "story" with everything that happens there. A lot of the rest of it won't fare nearly as well though so they can't cut it up too much. However this plays out, I don't foresee it being more than 4 releases (though I sincerely hope it is less). I'm willing to go through with this first one and see it before I freak out and judge from there, but there is a breaking point in this somewhere and if they manage to hit it, they're going to fuck themselves.
Having said that, I will say they are being far too aloof about what the plan is right now though and it's kind of bullshit. "How many 'games' is this going to be?" "We can't tell you because even we don't know lololololol." No. Not cool. How have you not already planned this shit out some time in the past decade you dumb fucks? I don't care if the first one is so extensive it takes 2 Blu-ray disks and that makes the potential for the rest a bit hazy. Sort your shit out. We don't have long before a new console generation too so logistics if this takes too long is going to be fucking miserable for both sides knowing how Square likes to function anymore.