usually determine the amount of fantasy allowed, the type and amount of sex, and then simply click new topic.
of course coming up with character sheets helps, as does a base discription of the world in question
That's certainly a good starting set of questions. I have a couple others that I think can go a long way toward helping define the sort of game people are after.
1) What does "Spartacus"-like mean? If we're going by the show, that implies a similar scenario to it, gladiators set up within the house along with other slaves; for a larger game, perhaps there are even multiple houses in the city, giving multiple layers of plot (owners' machinations, the intrigues within the houses, the arena itself, etc.). If instead we're going more by the movie or historical thrust of his story, then we're talking about a slave rebellion; in that case, the story is more about the war and its fallout. Either we'd focus on one side, dealing with its successes, problems, and consequences, or have an ambitious GM to oversee some cross-faction play between the two sides and facilitate the execution of the war.
2) Are we using a rule system or free-forming it? Normally, I'd say free form would be totally fine. However, if the exploits in the arena are going to translate to anything in the game, at least those characters should need some mechanical meat upon their narrative skeleton. Even if free forming, we may need to negotiate some sort of system to handle a few things. Namely, who's in charge and which characters get to be more of the 'big dick swinging' in the group. Having a Crixsus and a Spartacus vying for the top spot is good tension, but having every gladiator being a true paragon of the arena will be pretty silly.
If I didn't have a full time job that's calling for overtime now, I'd offer my services to manage this game, but I am interested in playing and would gladly help however possible. I happen to be an amateur Roman history buff, so I can help with cultural matters as well as the possibility of using fantastic elements which fit appropriately with the beliefs and myths of the people of that time, Roman and otherwise.