This may not be original, but I can’t sleep……….keep in mind I have not been on here a long time. My apologies if this idea has already been proposed 50 times in the same exact manner.
I have decided that the best twist to the end of LOST that falls within the “rules” laid out so far would be as follows: In the end, human life on the planet will cease to exist. No matter what changes are made, the end arrives at a certain date and time. It only ends once. The focus of our heroes and everything we have seen the last few years can only change HOW life ends. The difference will be whether life ends naturally (like, maybe the sun decides to go haywire and we all fry one day, or it burns out) or we kill one another through war.
As a result Jacob and MIB really are playing a game of sorts. There could be any number of scenarios. One possibility is that originally the U.S. military caused an accident when they were testing weapons. Jacob and his team of heroes were able to delay that outcome using neat tricks like time travel. But destiny always course-corrects; that’s just the way it goes – insert theory about that here - So what happens next? The Dharma initiative sets up shop on the island and they begin fighting with the original inhabitants of the island, then their experiments and the conflict combine to create a different version of the end of the world. Jacob intervenes.
In subsequent decades other things happen on the island, and they always conclude with a man-made method of wiping out humanity. A group of violent activists, lead by Alex, causes the end of world when they destroy the island. Perhaps a particularly greedy individual tries to exploit the island’s resources and creates some other kind of man-made disaster as a result. So Jacob has been delaying and delaying the end in an effort to prove we can die “naturally” and not by killing ourselves. He is delaying Mankind’s death same way Desmond delayed Charlie’s death. MIB, who along with Jacob can see flashes of the future, is just ready to let it end with our own stupidity killing us.
This ending would fit perfectly with the “bittersweet” description given by Matthew Fox. The writers have pointed to a “war” that is coming. In the end the heroic choice will be to avoid the war (I have seen a variation of that idea mentioned) and other man-made disasters. The perfect ending would be to ultimately have some natural event kill Adam and Eve with the full knowledge of their great accomplishment.