Okay, totally spit-balling....
But after the line "Don't get me wrong, I don't care about AVALANCHE or the Planet for that matter!"
.... then you are allowed to leave.
In fact, then Midgar becomes a complete open world. Potentially 300+ hours of missions, Arkham City meets Skyrim. Detective missions. Dangerous missions. Full exploration on top of the Plate. Night and day system.
All the while, though, Tifa is looking for you. Her AI is hot on your tail. When she finds you, a cutscene happens - rendered in real time depending on where you run into each other. Cloud remembers the Promise, and the next AVALANCHE mission becomes available.
I actually hate this idea?? For pacing's sake. Do you really think you would sit there and play like 100 hours of gameplay and then be willingly dragged back into the storyline of the game? Optional side missions in games are scattered throughout for a reason. You don't want that completely distracting from the actual game. You especially don't want to throw 300+ hours of optional content directly in the beginning of the game. So many people would never progress the story. And the story is so important in the type of game FFVII is. Damn near paramount.
You're talking about trying to cram an entire game into the beginning of a completely different game. This would be much better handled as two games. Or a DLC, but 300+ hours of content ain't DLC, that's a $50 at release game.
If they were going to put open world Midgar material in at all, I'd place it at the point in the original that would have been the beginning of disc 3.
They actually handled this really well in the original game, I think. There's some lil side missions you can do throughout the game, but for me personally, I'd hold off on every thing I could until the point where the game is like "We're off to the North Crater!" because at that point, if you have played video games and understand their structure, you know, okay, I've got one more dungeon and a final boss and then I'm done. Now I can totally ignore the manufactured sense of urgency, that giant meteor in the sky, and I can just go chill, revisit places, breed a gold chocobo, help the kalm traveler, play in the gold saucer, buy a villa. And know that at any point in time you can bop back in the airship and go spend a couple hours finishing off the game.