Hey, I can see it now:
Final scene of the series. Our heroes learn that in order to save the day/world/Island, one of them has to stay behind to flip the dead man's switch to teleport the Island into the past or something like that. Des volunteers because he's a hero and all, and Penny volunteers to go with him. They get teleported into the past on the Island, but will never be able to leave it or interact with the real world, forced to live out the end of their days. So they do so together, and finally die hand in hand in the cave. Or they would die almost right after transport.