I have a few suggestions you can use.
1. Natural disasters: Every now and then a disaster (tornadoes, volcanoes, earthquakes, floods and even a godzilla looking monster) and the city in the back ground gets destroyed and you have to rebuild the city
2. Payoffs: A barbarian horde will enter your throne room demanding gold if you pay them off they will leave but if you refuse they will get angry and the city in the back ground will catch on fire
3. Experiments: An alien will come up asking if he could abduct some of your people. If you say yes you'll lose 10 people and he will give you 200 gold. If you say no he'll say "Oh fine i'll go somewhere else" and walk away.
$. Zombie apocalypse: At a random point a bunch of zombies will show up and start eating people. Your knight comes up and asks if he can be the one to fight them off. If you say yes he leaves and beats the zombies but if you say no the zombies come in and the screen goes black and when it comes back you become a zombie and play as a zombie king.
6.Thirsty vampire: A vampire will walk up saying she's thirsty. She ask you if she cann drink the blood of some of your people. If you say yes she leaves and you lose some people. If you say no she gets angry and threatens to come back (she does too).