Space Marines in the Vineyard
I'm running this with the DitV system. The rules for gaining new traits will be a little more regulated to ensure our group can operate with them, and for purposes germane to the game's premise.
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Space Marines in the Vineyard
Marines work better for the DitV system than a lot of other options, I think; they have a command structure, and a nominal mission, so they generally have an incentive to get involved in a situation for better or worse.
Special rules would be as follows: a Space Marine automatically gets a 'super' trait, rated at d20 (oh so nerdy), called "Space Marine". If the player can make a good argument to fellow players and the GM that a Space Marine would be better in this given situation, he can roll it - it's just that when using it in raises, the player needs to have a suitably outsize description of what happens, like "I spit my Space Marine acid in his face, knee him in the groin, and pin him on the ground." You can't roll your Space Marine die unless it's a suitably dramatic action.
Conflicts
Space Marines isn't DitV; sure, I'm using the same dice mechanic and stats and so forth, but there's some critical differences, and I'd like to pinpoint them. First, DitV as a game is geared towards personal, relational conflicts. Our group traditionally hasn't dealt too much with these sorts of things, at least not explicitly in a dice mechanic. I'm scaling back the focus on those sorts of interpersonal conflicts, in an attempt to keep the arenas in which we roll dice a little more familiar.
A conflict is any time when, in the course of telling their stories, two parties disagree. You might say "I fortify the battle line with mines", and I might say "Sure," or I might, for reasons my own, say "No". When someone says "No", the dice come out, and we work it from there.
Note that as GM, I'm only gonna push for rolling dice if I think it's neccesary. If you're doing things that your character more or less has aptitude for, according to your concept, then I won't interrupt your flow. If your heavy gunner character wants to samba at the social, though, then we'll grab dice. You, of course,are free to bring out the dice any time.
Static Difficulties
For situations where your character is conflicting against "the story", ie the environment or forces of plot, you roll your appropriate Stats and Traits, and I roll 4d6 + 4d10. That's a pretty easy dice total to beat, given the scads of Dice available to most Space Marines, but it makes it likely that I'll throw a couple wrinkles your way, and you might have to take Fallout. Since I'm rarely gonna make you roll dice in situations like this, keep the potential for Fallout in mind when you ask to roll dice.
Experience
Since the standard DitV experience system is also not really appropriate, I'm revising it: any time you've gained experience, you can take a temporary Trait or Relationship that works for the next appropriate scene, according to the rules as they're written. Keep track of all such Traits or Relationships that you gain over the course of a session; at the end of the session, you can choose to keep one (1) Trait, and one (1) Relationship of those that you gained during the session.
Stakes and Earning Extra Dice
In most cases, I'm going to make sure we have appropriate Stakes defined, and I'm gonna push for smaller Stakes most of the time. You're free to haggle with me over the Stakes, though. Something I want to encourage is the use of temporary Dice as Stakes. For instance, say you want to set a trap for the encroaching Chaos Marines. I say "sure, you set a trap, you can add it as a Thing at d8 (it's a Big trap) for the next scene." You say, "I want that to be 2d8, cause I do excellent work" and I say "prove it; it's dice time." Those extra dice you wanted, I'm gonna roll those along with my 4d6 + 4d10 so that when (if) you win, you earned it.
I'll only ever make you roll for *extra* dice, though. Your plans should always be worth some dice; if you have ideas of why they should be worth more, we pick up dice and go to work.
Dramatic Large Battle System
Since a fair amount of SMitV would revolve around leading troops into battle, I've innovated the Dynamic Large Battles in the Vineyard (DLBitV) system. Mechanically, it's similar to the standard conflict resolution in DitV.
Phases
Battle is broken up into phases: Parley, Long-range, and Close. The Stats rolled for the various stages of battle are different though - Heart + Acuity for Parley, Acuity + Will for Long-range, and Body + Will for close range.
Each is a specific conflict, and the winner of an early phase takes bonus dice to the next phase. Any unused dice from an early phase may also be carried over into the next.
Giving in a phase means termination of the entire conflict and that fallout up to that point is calculated. A Commander can't Give in the Parley phase and then initiate Long or Short range conflict - a phase must be played completely through, until one side has no dice left.
Commander
The Commander of a force will always be a PC Space Marine, or an Important NPC. They use their same Traits, whichever applicable for the current phase of combat. Note that two PC Space Marines cannot join into one force, and that a force composed only of a PC Space Marine or NPC is completely viable.
Soldiers and Forces
Solders are rated in dice, added as Traits to the Commander's Traits, and rolled at the time they're brought into play. Raises using soldier dice must account for the actions those soldiers take.
A Force is a composition of soldiers, basically just the list of extra Traits your commander can use in battle.
For instance, if your Force had "Veteran Scouts 2d8", and you committed them during the Parley phase, you'd roll the dice then, and probably do some raises against the information your scouts collected.
Miniatures
Ideally, the battle is played out with miniatures, on a tabletop space. Every raise involves moving some of these miniatures around; if the raise is the Commander's and a general tactical maneuver, the Commander can move any of his models; if it's only from Forces dice, then only the Forces whose dice are involved in the raise may move.
Fallout
Fallout isn't calculated until after the battle(?). Dice of Fallout cancel out dice of Forces or act normally on the Commander, at the Commander's discretion.