You could do it with Conquest mode. The tricky part is coming up with buildings which would make sense. Maybe have a main base / castle per side, and then a blacksmith, stable and fletcher which control the equipment you can choose from (i.e. controlling the stable lets you buy horses, controlling the fletcher expands your range of ranged weapons and ammo), plus of course a farm or similar which generates money.
Would add a nice layer of strategy to the existing conquest, and of course you could remove the count down timer in favour of taking the opponent's castle.