I had an idea once for a rogue-like campaign game but somehow I never had the time to work on it. The game engine was going to allow a party of five adventurers (the player's character and four recruited allies.) It starts out with the character alone in the blasted ruins of a village. Overhead he witnesses a powerful uber-undead flying away. Before he is out of range, however, the player hears him call out something along the lines of, "You fool! You complete and utter fool! Now at last the world is mine!" followed by the requisite evil laughter...
When he starts searching around be finds a corpse that bears an uncanny resemblance to him (or her) self. There are several other corpses nearby and hordes of blasted undead, demons, and other evil whatnot. The only survivor in the village is the old seer, now badly injured from the effects of the battle. With her last words she say, "You! You must undo what you have done, or the end of the world is nigh! Seek ye the seven arcs; travel forth into what must not be. Only then can ye return hither with the answer!" With a groan she passes away.
So the scenario is that the character (and allies) traveled through time but in so doing they unleashed a horrible monster that will bring doom to the world. The character must wander through the world of the present, seeking seven parts of a metallic ring. The ring must be assembled at a specific location where the ley lines intersect. This will allow him to travel forward in time to seek an artifact that can destroy the evil undead creature. (Finding out this is requirement is one of the components of the quest.) The undead overlord is well hidden in this point in time, working its will slowly and cautiously. Should the character find it, however, he or she will be unable to destroy the monster with any weapon then in existence.
The world of the future is a horrid, undead place where the living dwell in hidden enclaves and the land is inhabited by all manner of undead monstrosities. (There are several cities of undead, each competing for domination of the land.) Within that world an artifact was hidden that can destroy the evil undead overlord, but it can only be used by the living. It lies within the heart of a great tree, protected by a powerful but mad druid (the creator of the artifact who used it to mutate the tree into immense size). When the artifact is retrieved the gate can be used to travel back in time to tackle the overlord. As they return to the gate, however, they are pursued by packs of undead. They manage to jump through the gate, but before it closes the undead overlord leaps through, accompanied by a multitude of allies.
In the final battle, either the evil undead and its hosts are slain, or else the undead overlord steals the artifact; hides it, and begins anew its plot to take over the world. (In the later case, the scenario begins back at the beginning with the same starting character. (Although, of course, the player can start the game over again with an entirely different character.
Well that's it. So what do you think?