I've tackled a lot of Magic-related topics in my speculative articles. I have done ones on the next Commander set, on possibilities of a future block name based on a trademark Wizards of the Coast registered, and on a sixth basic land type. But I've never tackled the true bogeyman of all Magic rumors, the one thing that is speculated immediately literally after every new set or block is announced whether it is remotely plausible or not: The sixth color.
Usually referred to as "purple," an additional member of Magic's color pie is perhaps the ultimate shocking twist a designer could use. R&D has made attempts to almost add another color of mana (Coldsnap's snow mana and Rise of the Eldrazi's, well, colorless Eldrazi), but neither of these really game close to the true definition of the concept. Magic players have been bandying about the idea since time immemorial; the first great Magic urban legend, Throat Wolf, was often stated to have a purple mana cost.
The problems with purple are twofold: One, almost every conceivable flavor and mechanical philosophy is covered by one of the five existing colors. Two, the addition of a sixth color would throw off the delicate balance between each of the colors as equals with two "allied" and two "enemy" colors.
The most-often-proposed solution is having purple be the color of a kind of zen, surrealistic philosophy, totally alienated from the other five colors. Mechanics proposed for purple often have to do with enchantment manipulation, dealing with the exile zone, shapeshifting and face-down or morph creatures, and other things that seem "immaterial." MTGSalvation user Tahazzar, who often makes custom cards incorporating the sixth color, has an excellent masterpost regarding the most-often-articulated versions of the purple color philosophy.
The other way of dealing with a sixth color is much more radical: It dares to propose throwing out the color pie - or at least allowing an entire parallel one to exist - for an entire set or block. This is, to say the least, extremely audacious. It would be the single most divisive and controversial move ever made in Magic design. For Mark Rosewater's part, he seems to believe that having five colors is a necessity, as when Tumblr user vixorien asked him, "With the color pie being so crucial, do you think it would be better or worse if Magic started with a different number of colors? Is having an odd number of colors optimal?" and he answered:
I believe five is perfect. As far as I’m concerned, Richard (Garfield) nailed it.
This, of course, does not preclude the "alternate color pie" theory. Another great MTGSalvation thread runs down the community's consensus about what those colors would entail, as user HGaramond puts it:
Purple: Imagination, dream, fantasy
Yellow: Individualism, profit
Orange: Realism, inevitability
Brown: Industry, greater good/collectivism
Pink: Compassion, community
These are all somewhat similar to the ideas embodied by the "true" color pie of white, blue, black, red, and green, so they are familiar ground to build mechanical identities around. Yet they're also pleasingly different; I would love to see an alternate color pie block (or, let's be reasonable - a third-set twist) using these colors.