Mirrodin block first came about because I really wanted to do an artifact block. Invasion had done multicolor, Odyssey had done graveyard, Onslaught had done tribal—it was clear that one of the biggest themes we hadn't done yet was artifacts. I've talked about this story several times, but I've never told a different story about how Tyler Bielman, then head of the creative team and Mirrodin design team member, and I first sold Mirrodin as part of a three-block meta-story.
The Weatherlight Saga had wound up in Invasion block and while that didn't quite go the way I wanted (a man named Michael Ryan and I were the ones who originally pitched the Weatherlight Saga—but that's a story for another time), I was eager to try and tell another large multi-block story. Tyler shared my passion for larger story arcs and he and I came up with a plan three blocks in the making.
Block One was to be an artificial world of metal made by a Planeswalker who we called the Zookeeper. The Zookeeper had populated the world with mechanical creatures of his creation. He then used a device to pluck creatures from other planes and trap them on the metal world. The newly acquired inhabitants would have to fight the metal creatures to survive. The main character of the story would be a champion plucked from another world who figures out that the world is artificial. She fights her way to the home of the Zookeeper, intent on putting an end to this mad experiment. When she meets the Zookeeper, he tells her, "I was wondering which one of you would make it here first. Come, we have much to do," and then they both disappear.
Block Two was set on an underground prison world. (Note that this was many years before Scot Van Essen would propose that idea for his world in the Great Designer Search 2.) Besides the prisoners, there are numerous creatures native to this underground world who are also enslaved. The warden (who we called the Warden) is a harsh man who forces all the prisoners to fight one another in arena-style combat. The main character of this story is wrongfully imprisoned. While trying to escape, he comes to learn that many of the prisoners are likewise wrongly imprisoned and he realizes that the Warden is up to something. The protagonist ends up having to fight in the arena. When he wins the competition, he finally comes face to face with the Warden. He tells the Warden that he knows something is going on. The Warden replies that he's just scratched the surface.
Block Three is a violent storm world. It is the neutral plane for a giant conflict between the forces of the Zookeeper from the first block and the Warden (who we also learn is a Planeswalker) from the second block. This block is an all-out battle between the metal creatures and kidnapped creatures from the first block against the prisoners and underground creatures from the second block. The winner of this epic battle would win some cosmic prize that would go on to threaten the Multiverse in the future. (We hadn't decided exactly what it was.)
You'll note that pieces of the setting for the first block remained. It was still an artificially made artifact world and it did trap creatures from other planes, but Brady Dommermuth, the man in charge of worldbuilding for Mirrodin, chose to advance the story such that the creatures from other planes had adapted over time and become part of the metal plane's ecosystem. Gone was the "survive the elements of metal world" from our story.
So what happened? Well, the Weatherlight Saga had not gone as well as anyone had hoped and everyone was a bit gun shy of making a commitment to another multi-block story. What if one year in we learned players didn't like it? We'd be stuck having to make blocks that might not be wanted.
That is the tale of the three-block story that never happened and the one world from that story that went on to become one of the most popular settings for Magic.
I'm actually kind of sad this thing didn't get more traction. It sounds like it would have been pretty fun.