The content wall is struck if the player has done/built everything, and the game no longer does anything to challenge the player or endanger the village. At this point, the game becomes purposeless and boring.
Responsive and dynamic systems driving things like weather, monster spawns, and other things that force the player to react to the game.
The actions the player takes could influence the in-game systems (i.e. the world), which will in turn cause the player to have to react the the changed world. In other words; the player does something -> the game responds to the player's actions. The game already has the inverse relationship of; the game does something -> the player responds to the game's actions.
Unfortunately the challenges the game presents are currently predictable. Once a village has survived to a certain size there really isn't anything to worry about; you've hit the content wall and nothing will endanger the village. However, making the game dynamically react to the player will cause the game to be less predictable, and exponentially push back the content wall.
Basically, the in-game world itself could use an AI. Then the game wont only consist of the player reacting to the game. The game will also react to the player.
Here's a little text diagram to illustrate what I'm saying;
How the game is now;
Game presents problems -> Player solves problems -> Player hits content wall
How the game could be:
Game presents problems -> Player solves problems -> game reacts to the player's actions -> Player solves emergent problems -> game responds to the player's actions -> and so forth.
Theoretically, this could indefinitely prevent the player from hitting the content wall.
Implementing this sort of circular causality could potentially abolish one of the game's central problems.