Looking for at least 4-6 players.1 Lone Antihero.
Feel free to take the reins of creativity from there.
1-2 Lawmen, ranging in flavor from honest local lawman
or an ammoral Knight Templar federal policeman
2-4 Bandits, part of a group of a larger group of NPC bandits (at the start of the game, at least), that can run the gamut between petty, stupid, but extremely competent
to Philosopher Killer
to the good old fashioned Bandito.
But it's all up to you.
Since I prefer to discretely just drop a concept on the ground and back away from it slowly, letting you the players do whatever you want with it... consider the above as merely suggestions.
See, Spaghetti Westerns originated as a deconstruction of the typical 'Heroic Cowboy' genre of westerns... but recently with films like True Grit and games like Red Dead Redemption, the genre has shifted once again in the deconstruction territory, becoming even grittier and 'real' than the old Spaghetti Western way of doing things.
I like grit... but not TOO much grit. I want to bring back the Spaghetti Western. More this.
(Even though both songs are pretty awesome. <_<)
...The first scene will take place on a train, I've decided.
Anyways, down to the nitty not-as-gritty!
Rating? Well... The whole story could go by without any explicit sex at all and I wouldn't be dissapointed any. Your call.
Systems or Freeform? I'm inclined to say system... just so there's a definable way of allowing the lone ranger to gun down 4 or 5 NPC bandits with ease, the lawmen to rally up an effective posse, and the bandits to say... steal from the US Army supply depot. -and to heighten the tension whenever the three groups inevitable collide. Which I hope they will... repeatedly and non-fatally, if I can manage it.
Also, if I do a decent job as the DM, the three groups may end up by circumstance cooperating with and against eachother to accomplish goals. I want to stage a situation in which all three factions are motivated by a single object/person/thing but have entirely different reasons/methods/circumstances surrounding their pursuit of said macguffin.