Tons. You can just about make them up.
1) Astrologers and seers find out that the world is about to: Collide with another world/star/asteroid/etc. Intersect a plane of madness/darkness/pure evil/etc. The heros have to stop it, or all is lost!
2) An ancient lich or other immortal terror has been free upon the land! The PC's must find it and destroy it! The twist is that the Evil One has assumed the likeness of the world's most power empire, and is even now preparing its plans for conquest! Simply defeating the thing won't be enough, they must expose it for what it is, or risk a war that will engulf all the world and lay waste to the land!
3) People around the world are starting to change into terrible monsters and attack unchanged folk! The PC's must fight hordes of powerful nasties, gather the survivors in a safe, or at least fortified location, then find out why and how this happened...and stop it.
4) The PC's have been chosen by a pantheon of new gods to help them create a new world. The major work's done, all they need is some help exterminating some of the previous inhabitants (hideous monsters from Before Time and such), and some leaders to help the new fledged races find their way. Of course...the bad gods invited some people over to help out too...