Roaming encounters -
These could work like random quests that have a low possibility of popping as we snail crawl our way across the realm. Anything from defend yourselves againt the roaming band of wolves/orcs/undead/bandits to defend the nearby village to hunt down the murderous boss like werewolf. Rewards could be standard or unique from simple gold or crowns to unique special units like the thankful healing cleric or fighting monk who joins you to repay you for saving his church ect...
Border wars -
An ambitious possibility giving more purpose to our petty skirmishes and sieges. 3 factions contending for control of territory. Every siege against an enemy faction pushes control of territory in the corresponding direction. More control means slight advantages in rates of resource gathering and maybe random free units available for claim to the victors.
Npc war sanctions -
At war? Do something about it! Npc towns we are war with periodically would send envoy's to demand tribute, parlay for peace or threaten imminent attack, which would be represented by a slow moving army that could be met on the field or awaited to defend at the city gates.
Natural disasters -
Very rare occurrences that could affect entire kingdoms, earth quakes damaging buildings, sinking mines, raising new ones. Tornadoes tearing up the town and making off with our precious cows.. Magical storms. Mutating our peasants for a few hours into powerful crazed wizards. Possibilities are endless.
Just a few thoughts