It's a parable, nothing more or less. Simply a thoughtful illustration depicting the responsibility of a Christian and the blessings that one will recieve for carrying out those responsibilities. It also depicts that love and faith would be the primary components for success in accomplishing their duty as Christians. To view "who is the faithful & discreet slave" as something prophetic with a need to figure out who it is specifically applying to, as opposed to a question that is illustrative or rhetorical, would be like viewing Kennedy's question "ask not what your country can do for you, but what you can do for your country" as prophetic.
I think a even better way to illustrate it would be the question commonly asked, "If someone told you to jump off a bridge, would you?" Seriously, if someone asked you that question, you wouldn't think to yourself, "I wonder what bridge he was referring to? The Golden Gate bridge? The Brooklyn Bridge?" No, it's not about a specific bridge!! It's about the thought behind the action of someone jumping off the bridge along with questions pertaining to naiveness, common sense, stupidity, intelligence, etc..