While there might be a prisoner or war system, which is akin to slavery in the sense that people are kept against their will, other systems are less likely. There is a good post by Caspian about why it doesn't really add much enjoyment to the game which is found here, which ends:
"All-in-all, I can't think of any player that would truly get joy out of a slavery mechanic.
As I said before, as a PoW mechanic, both the Achiever as well as the Killer might "accept" it, if it's done properly."
Personally I think that it is the sort of system which sounds a lot more fun then it is to play. After all, with a game there is a very easy way to escape slavery, log off and don't come back.