I'd be surprised if the overworld wasn't in this game, likely as an endgame area. They are promising an amount of content comparable to a normal FF, so I cant imagine it being desirable to set that all within the city.
Point is, they need to make it a standalone experience to draw in new players, so giving people an overworld to tool about in after escaping seems like a good idea.
I think the whole game of Final Fantasy VII Remake (part 1) will have a total exploration map of Midgard.
This implies that all 12 sectors of the Midgard mako reactors will probably be explorable.
And what better way to go from one sector to another, if not with a motorcycle.
(or even by train)
I am almost certain that the bike will be one of the main means of transport of the game, to visit areas and sectors of the game.
(and all Avalances will probably use it)
So, they will create a solution that, during the last events of the Midgard escape, will give you the possibility to play all the secondary ones that the game will offer you.
(or something after the events of the escape, but I don't think)
Midgard will probably be treated as a big city literally openworld, with history, dungeons, events, missions, and many new events for its universe.
In addition, of course, to all the events of the classic original story.