Oh, I don't imagine they'd possibly expand every part of the original game to the degree that they did Midgar - even not knowing exactly how long it was, I could confidently guess they'd be working on it for the next few decades if they tried something like that. Midgar getting this much expansion probably just fit conveniently for them narratively (establishes the characters, world, and main plot quite thoroughly, while still having its own satisfying narrative arc and conclusion to it), and allows them to wait to do the larger game world for the next console generation, while having the assets (and engine?) they made for this title as a starting point to work with going forward. I'd guess we'll see maybe two more parts, assuming they will still be doing a fair amount of expansion on the original material.