Let's have a look at the story beats outside of Midgar that they need to hit:I don't think you read the OP at all. Expanding one area doesn't mean expanding all areas to the same density and making it anthology.
Midgar is the best part of FF VII and other areas can be shortened if anything.
Sephirot back story
Chocobo ranch
Turks, lots of Turks
Junon military parade
Barret's hometown
Red XIIIs hometown
Destroyed reactor village
Cloud and Tifas hometown
Gold Saucer
Meeting Cid
Meeting Caith Sith
Meeting Yuffie
Meeting Vincent
Yuffies hometown
Temple of the Ancients
THAT moment with Aerith
The whole freaking ice area
Truth about Zack
Escape from Shinra
Cloud breakdown
Getting Huge Materia
Fighting Weapons
The Promised Land
Fight Sephirot.
Here's what happens in Midgar:
Blow up a reactor
Meet Tifa
Go blow up another reactor
Meet Aerith
Don Corneo
Bye bye Avalanche
Storm Shinra
Bye Midgar.
Sure you can add and expand but Midgar being it's own game is ridiculous as either a lot of the future plot points elsewhere will have to get cut or we're looking at way more than 3 games.