I think you have a brilliant idea with the waves there. If pressure would increase, decrease, in somewhat predictable ways, it would give days of pressure and destruction and days of rebuilding and hope. It would be easy to implement something like a sine wave of monster activity.
For example, as a quick experiment,
y=sin(sqrt(x*10)) * sqrt(x) + (x/10)^2 + 2 gives this nice curve. I'm sure someone with more math background can improve upon this.
