In a setting prone to both retcons and time-travel shenanigans, with planeswalkers capable of crafting spells of world-altering power, very few broad storyline changes are "final" in Magic - but the fact of the revived Phyrexian host taking over Mirrodin at the end of Scars of Mirrodin block seems like one of them. In point of fact, both possible endings for the second block on the metal plane were announced, Mirrodin Pure as well as New Phyrexia - but the future was predecided, the ending was known early in block design, and we all know which one became a reality. And since given the length of the set design cycle (two whole years), having the audience vote on the ending of a block would be unfeasible, meaning Mirrodin Pure really was "what could have been but will never be."
However, Magic players are nothing if not damnably curious and hungry for variety, and when electricsundials asked Mark Rosewater on his Tumblr blog, "Couldn't you release Mirrodin Pure as a supplemental product? Something tiny, like duel deck sized, just to show what could have been?" the R&D head decided to open the question for discussion.
The alternate-universe precedent of Planar Chaos seems to mean that such a thing would be a probable future product, though most of the opposition to the move stems from the failure of Coldsnap as the erstwhile-supposedly "lost" final set of the Ice Age block, and of general dislike of what-if scenarios as played-out or uninteresting.
Overwhelmingly, though, the 74 responses so far have been bullish on the idea. Users seabutcher - "Maybe as some kind of weird alternative-future time travel themed block (re: Time Spiral, maybe a crazed Koth finds a way to time travel, which saves Mirrodin but also threatens the temporal stability of everything?)" and luminousumbra - "Definitely. If nothing else, it would be neat to have them show up as alternate history cards in a Time Spiral-like thing" offered similar suggestions for how to do it, while vamspapi's idea was logistical: "I would love to see it as a “microset” released - smaller set size, 12 pack boosters?"
All in all, with only three variant-format product lines to fall back on - of which only Planechase and Commander are considered successful - R&D is probably thirsting for new ideas for summer supplement products, and Mirrodin Pure is a definite possibility (though it wouldn't appear as an ordinary set given the desire not to reverse the consequences of New Phyrexia on the storyline).