Event/quest idea: endless invasion
Unlike the typical invasion, the endless invasion keeps spawning hordes of monsters from the dungeon until you root out whatever is spawning them. Imagine: to solve this quest, you have to leave the town; on the other hand, if you leave the town, you risk getting the townspeople killed. You can choose to build a totem if you want, but it's still risky. What do you do?
Even better: there is an evil altar making the invading monsters 1.5x more powerful. Do you destroy the altar first - since there's only one of it - or do you try to source out the 3 Spawning Gates? What to do what to do!