Alright, been thinking.

The RP begins at a standoff.  There are two groups: one who is loyal to the current King, and one who resents the King.  I'm not sure what the disparity is between these two groups, but I'm looking to Wars of the Roses for inspiration.  The rebels have gathered their forces and marched towards the capital, but the King has assembled his loyalists and marched forth to meet them.  Two encampments stand at opposing sides of a mountainpass, and a battle is about to occur.  However... a parley has been accepted.  The rebels are invited to peace talks with the loyalists.  The goal of the rebels is to negotiate terms, while the loyalists must try to do what they can to prevent bloodshed or whatever - your goals are your own.  And then we go from there.

One important thing to note is the Paragon.  Each faction has a Paragon, and this is essentially the strongest and most important 'piece on the game board', so to speak.  The Paragon is extremely loyal, probably a son or close friend; he can fight well, inspires his soldiers, and leads like a true commander.  He can turn the tide in battle, and could mean the difference between victory and defeat.

Perhaps the Royal Family has been killed and now two rival lords are vying for the thrown; you must pick a side - or make your own side, up to you.  We can do a second plot concerning the princess and her loyal retainers as they escort her to safety, and perhaps she can rebuild an army later on.

I was considering resources.  Supplies, Coin, and Food, and each faction produces one.  You need to secure alliances or conquer land to acquire the rest.  Run out of supplies, and you have no weapons to fight with; run out of coin, and you've no money to pay your troops; run out of food, and your men starve and desert.  Perhaps each faction has 4 territories, so have +4 of their resource; losing a province decrease that to +3, while that +1 which was lost goes to the conqueror.  Otherwise, you could initiate trade and you get +1 of their resource per turn, and they get +1 of your resource per turn.

Or perhaps everyone produces all three resources, some just produce more than others. And I like the idea of Traits.  Maybe one faction is known for powerful knights, another known for poisoning people in the past.  Maybe you have a powerful fleet, etc.  I'll allow X amount of traits per faction.  Everyone may have about 5000-7000 soldiers in their army to begin with, so merged armies of 2 may exceed 10000-14000, and on and on as they gather more allies.  This is your starting force; you'll never have this many men again in the future.