I've been pondering over this question for a long time and so I've decided to write my thoughts on it...
A brief intro about Zack. He's the main character in Final Fantasy VII: Crisis Core, a SOLDIER 1st Class and a friend/mentor of Cloud (who is the main character in Final Fantasy VII). Zack was a person who upheld personal honour and his ideal dream was to be a hero. His former mentor, Angeal (another SOLDIER 1st Class), had passed him his family's Buster Sword when Zack 'set him free' from his monstrous side. In the later parts of Crisis Core, Zack and Cloud were hunted by Shinra for they had escaped a lab experiment. Attempting to flee back into Midgar while remaining undetected by enemies (which failed), Zack supported an unconscious Cloud, who was suffering from mako addiction. In the end, the Shinra Army caught up with them and Zack gets killed as a result (it's always a tearjerker to play this part of the game). Zack passes his legacy to Cloud in the form of the Buster Sword, formerly the treasured blade of Angeal. As Cloud wields the Buster Sword, the plot continues to the original 1997 Final Fantasy VII game.
The question now... is what if Zack and Cloud weren't discovered by the Shinra Army? Before the cutscene of the army finding Zack and Cloud, we see that Tseng of the Turks (an Shinra-employed force used to do underground work) telling his colleague Cissnei (another Turk, unmentioned in later plots) to find him and bring him back... to save his life. Cissnei willingly do it not just for the job, but also likely because she had a crush for him (it's too obvious if you played the game). Tseng said that he still had 88 letters for Zack (Zack only received the 89th. They were written by his own love, Aerith).
Now.. if Cissnei managed to rescue them in time, and ensured that Zack and Cloud were kept in safety (probably somewhere in the slums), Zack would probably be able to spend the rest of his free life with Aerith. The mako addiction in Cloud may not have triggered the overwhelming confusion which led him to live thinking he was Zack (he had believed himself to be in Zack's shoes in the earlier parts of Final Fantasy VII). However, if Zack had wanted to live his life in freedom with Aerith back in his hometown, Gongaga, he may still pass the Buster Sword to Cloud, so as to break free from his SOLDIER life. On the other hand, I don't think that they would move to Gongaga, because Aerith loves the flowers that she can grow in the Church of the slums, and they had wanted to sell the flowers together and spend time with each other.
The only problem is that the Turks are watching over Aerith (the last Ancient of the planet, I don't know how they found out lol), and should Shinra attempts to catch her, the Turks will have to follow the orders (which really happened in the original Final Fantasy VII). Zack will have to pick up the Buster Sword once more to defend Aerith (in a way it's like Cloud's role in Final Fantasy VII).
And what would happen to Cloud then? Zack only knew that Cloud was from Nibelheim, and it would be suicidal to return to it. Maybe he will let Cloud stay with them in the church, or let him wander on his own.
These are just my thoughts for now, on what if Zack hadn't died.
