I'd like to at least explore a bit of the world beyond Midgar, preferably, as it would give a good look at how the world would be handled (and give a good difference in location to keep one immersed/engrossed in the first game instead of just Midgar's dark streets).
If you wish variety, it is still possible to offer it while you're still just in Midgar. The answers to that are, IMO: 1/Implement day-night cycles and weather 2/Allow exploration on the top of the plate 3/Allow exploration in ALL sectors of Midgar 4/Give to each Sector a distinct "flavor" - after all, Jessie points out in the OG that each sector used to be a town of its own.
Still, I think that having a glimpse of what it would be like outside Midgar would be detrimental to the structure of the "Part-I" of the game.
Your looking at 25-30 hours of core gameplay in Part 1 if they expand upon the game like they've promised. Part 2 will probably only offer 15-20 hours of core gameplay, but because its the end game and it will offer you full access to the entire world along with dozens of hours worth of side quests I dont think people will mind.
As I mentioned before, IMO it doesn't make sense to make gameplay time estimates for the remake based on the corresponding time you spent on the OG. The scope is going to be really different. If Kitase and Nomura are true to their word, I really doubt that exploring Midgar will be just a couple of streets in Sectors 5, 6, and 7, for instance. Not to mention that most of the gameplay time estimates we tend to have for the OG are those from people who know the game by heart, without that sense of "discovering a world for the first time".
I tend to believe that, for a remake, we have to distance ourselves from how the OG was constructed (which was a product of its own time, IMO), and think how a new game (or, in this case, a new series of games) would be crafted.
When Peter Jackson decided to adapt LOTR as movies, he didn't hesitate to deviate from how the books were structured, for better results. IMO this was also the case of a story told as "multiple parts", and I don't see what was the harm in that.