Counter-invasions
When faction X invades the land of faction Y, there will be a random chance for faction Y to launch another invasion nearby to protect its land. Some factions will answer to threats in different ways, for example:
One does not simply walk into Mordor
This quote already says how dangerous and powerful Mordor is. So, if Gondor launches an invasion in Mordor, there'll be 100% of chance for a Mordor counter-invasion to happen. But if the opposite happens, Gondor may not launch a counter-invasion in time.
I'm not going to enter in much details about which factions can launch counter-invasions and which can't, because it is supposed to be an uncommon thing.
Also, imagine taking a stroll in the woods of Ithilien and then suddenly BAM, Mordor invades and Gondor counter-invades: Massive battle out of nowhere!
Patrols
They work like invasions, but on a smaller scale and they are defensive measures. To make sure their lands are safe, some factions will launch patrols, spawning a small quantity of troops that will roam around in search of enemies to fight.
When these spawn, a message will appear on the chat: Faction X sent a patrol nearby. But no horns! These patrols consists of only two waves of troops, to retain the small scale I mentioned earlier.
Foragers
This suggestion regards mostly the Moredain, the Dunlendings and other factions that don't grow their own food.
Like the Patrol mechanic, forager parties will spawn in fertile biomes. These forager parties are just civilian NPC looking for food. If possible, it would be interesting to see them breaking bushes and killing animals. And what does this means? Protect your crops because they are going to steal them! Exactly, once they break or kill something, they steal the dropped item.
A message appears in the chat when they spawn: Faction Y sent a rabble to forage the nearby lands.
Raiders
They'll drink from your skull! Orcs, Dunlendings and random evil-doers can spawn like invasions near you in four waves. This will bring controversy but, here it goes: They can bypass banner protection (but the giant sword icon spawns outside of the limits, you can breath now.). There will be torch carriers who will set wooden blocks on fire, depending if they're near it and there is a chance for it to happen.
Like bandits they will steal things but not from you, from chests! So make sure to guard your homes from them.
PS: I'm sorry for grammatical mistakes, I typed this in a hurry for no apparent reason.