Excluding the city burning to the ground, I had a couple others.
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Say a plague hits your town, and people are getting sick (can be cute sickness if need be). Your Herbalists are doing their best to keep up, but they need you to send a group off to get the cure. This then entails a Dungeons & Dragons like quest to get the Dragon’s Drool and return it to the village. If they fail, then it takes even longer for the Hearthlings to eventually recover, thus killing productivity for a while.
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Off in a distant land, Ogo has destroyed a neighboring village, and they have now shown up at your door asking for help. If the player agrees, they set their own small camp up on the map, to which eventually they start stealing from you the same way the goblins do. Being they’re Hearthlings and not Goblins, you have the choices of letting them keep take your supplies, Confront them about it and find a peaceful solution that involves you giving up hard to come by items you can’t reproduce, posibly even a chunk of the map. Forcing them to leave, or forcings them to join your town.
A couple small ones:
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After getting enough cultists together, and doing what @Solus said, they eventually begin demanding a church be built for them, and thus new blueprints unlock.
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Say you chose to be a trade town, and eventually grow big enough that the Herald (can’t remember Raaya’s equivalent) takes notice, and begins asking for a steady supply of products rather than a finite numbers. Upon holding this supply for an unspoken amount of time, new venders, or new NPC Shops that the player can place in their town become available.
I’m sure I can come up with more later, but currently, I’m running on 22 hours of no sleep, so I’m heading off.