I personally really just want the ability to enlist in a lords' army haha.
I like the idea of being a common soldier in service to a lord, but in the WB mods that have done it, it's always incredibly boring. You attach yourself to a lord's party and then the game becomes a fastforward map staring simulator with the occasional rare skirmish or battle, effectively not letting you play the game. I suppose the boredom reflects actual marching around or standing around garrisoning a fort, but that's hardly entertaining game play.
I'd like to see this done so that upon joining an army you are made member of a unit, say, 5-10 men with names and personalities and then the lord sends your unit on missions like foraging, reconnaissance, tax collection, bandit hunting or just guarding a bridge. Even fairly mundane military tasks can serve as the setting for an interesting quest with RP possibilities. If the lord's party actually goes into action then you'd fight with the guys in your unit, who might get hurt or die. If the lord's army is defeated, you could choose to desert with your remaining friends, who then become your companions.
Of course this would take a lot of work for what I imagine is a pretty niche desire so it's likely to remain a wish unfulfilled.