I love how in the Lord of the Rings there clearly has been a long buildup of tension.
So it would be cool if monsters, and especially wastelands, would slowly turn more aggressive based on events, how much you (or other players) harass/pressure them or just time passing. An event could be to get a mission you must complete to calm down a powerful wasteland (destroy the ring!). It could also be funny if you could on purpose harass a wasteland close to your enemy. With the risk that this backfires, and the wasteland turns against you.
Whatever is done, I hope there will not be a constant flux of irrelevant monsters. That just annoys the player. As much as possible, every battle fought should be meaningful in some way. It should not be common to get attacked surprisingly by a powerful monster. Especially in the early game surprise attacks by powerful monsters can decide the game without the player having a chance.
Most of the time the player should be informed (or he should be able to guess) that an attack is coming. There should be some way the player can know why the monsters decided to attack.
There could be this event: "The orcs are gathering in the mountains. Maybe we should send some patrols there?" -> The buildup of orcs start -> If the player does nothing, the orcs start to wander (and pillage) the farmlands -> If the player still does nothing, the orcs start to attack units outside cities. -> cities will be attacked. Clearing the mountains could be a rewarding, but challenging mission.
Similarly if there is a bandit camp, when you stay away of it, they stay away of you. When your borders start to get close to the camp, they will get a little aggressive (attacking units nearby), when the borders begin to pressure the camp, they will try to get rid of that pressure (more pressure, more chance of all-in attack against the city generating the pressure). Same could work with wastelands: if you give them little pressure, they will occasionally send scouts against you. When you pressure them more, they will start to fight back. If you attack them in force, they are likely to attack you back in force. Don't go harassing those dragons, they are easily annoyed!