It's possible. I tend to think that decision was made well before the game was thoroughly plotted out, including likely before a decision about Zack's inclusion in the ending was made. I think it was more on the basis of how iconic the setting of Midgar is for FF7 and possibly because many people always wondered what the original game would have been like if it was all contained to Midgar (I know I did), even if they were more wondering about further events in Midgar rather than stretching out the Midgar events of the original game to the length of a full game.