Is there a function or system which is time depending in which the effort required to brute force the decryption increases with time? It is easy to break encryption from many years ago because computers are so much faster. Is there a system in which time is a component and as time passes then it gets harder & harder to decrypt? Perhaps there is a physical entropy component which degrades after time? Something akin to disappearing ink, but digital.
Any protocol with long-term security becomes harder to break after the protocol execution has finished.
In the Bounded-Storage Model, protocols become harder of break as [information about the
randomizer that's not stored by the adversary] is lost. $\:$ (This point is similar to minar's observation.)