Who said the best you can do with MnB, a medieval game, is to create a musket mod? Now let me present you WWII China Battlefield, a total conversion mod of MnB Warband based on east Asian theater of WWII. Rifles, SMGs, MGs, grenades, mortars, even flamethrowers working smoothly and efficiently in the mod. There are 7 classes troops for each factions(rifleman, storm trooper, cavalry, machine-gunner, coehorn shooter, medic and pioneer), you need them all working together to win a battle against human players or waves of AI attackers.
Next should be some more caps, and guns.