Wouldn't it be neat if you could buy slaves, train them with time and expertise and then put them out on the market to the highest bidder? It could add a new dimension to the game.
I played a game called Motor City Online a ways back, it was a racing game but you could also build race cars or show cars to your own spec. Some players enjoyed just racing to win and gain money to purchase better parts or cars, others though just built amazing performing cars and sold them off as their niche in the game.
I find it an interesting twist to the game, but my ideas are usually torn up, thrown to the floor stomped on and then lit on fire as I am led back to my training yard where I take my frustrations out on my slaves by whipping them twice as hard for losing to gladiators ranked below them.