Theoreticly you do play as zack when Clouds telling the neibelheim reactor story in kalm, but he's messed up and thinks it was him. Would be call to propally play as Zack thougth, theres probably gonna be a spin off with him as the main character like Dirge of cerberus has daone with Vincent. Personaly I would have most liked to play as Jenova before she was locked away in staysis! what havoc she would have caused........