I think a good idea would to be able to Select Different Locations. This way you can have New Scenery, Landscape and New types of Sharks. By having the players able to select the location, you can use this as a way to Change the difficulty by picking an area with either More Aggressive or Less Aggressive Sharks. To keep the game refreshing you can also add levels with multiple little sharks, an Easter-egg with a Jaws sized shark that can take out multiple foundations at once, and even a shark-less water for the guy that just wants to build.
After seeing the 1.0.5 update I get the feeling you might be going a warmth meter or something. If you are or aren't you could also make a Location in the north where it snows frequently and the player has to factor warmth more heavily. The Possibilities are endless.