With D&D you would first have to say which of the universes you want the Mod to be in.
But actually all of these have big factions and wars as an integral part of their background story.
Greyhawk would be nice as its geographicaly factionwise very small in scope.
Forgotten Realms would be quite hard, as it encompasses almost the whole real world with lots of different cultures. Might be doable with a specific part of the world however.
Dragonlance might be a bit difficult because of the dragon mounted knights.
Dark Sun would be very cool, with the Dragon Kings as Factions and large armies of their warriors - though the inherent Psionic Powers of each soldier would have to be done with a little bit of imagination.
Birthright would be perfect, and if I had the time and skill to mod (of both I have only little) then the Birthright Mod fpr MTW would be finished already.

But there is another great (less well known) fantasy setting that would be interresting for RTW and thats Earthdawn.
Sadly nobody had that idea already.