Rift Cube

A big issue I've seen with people trying to give Purple an identity is that they keep calling it "Wisdom" and "Balance" and whatnot. They keep trying to place it above the other colors, in that it is somehow better, or good. But that's not how the colors are given personalities, ethics, alignments, whatever you want to call them.

White is not Good, and Black is not Evil. Blue is not Smart, and Red, Brutish. They have ideals that are more complex than right and wrong. Black can be selfish, sure, but sometimes it is necessary to know when something is not good for you and try to care for yourself. Red is chaotic and emotional, but its also creativity, spontaneity, and freedom.

They all have Good and Bad aspects, but it all depends on your point of view. Instead of working on Purple from Purple's point of view, you need to make Purple from Green's point of view. From Blue's point of view. From everyone's point of view. Purple needs virtues and flaws. It needs to represent an ideal that can be good and bad. I think that is the best starting place to design Purple.

EDIT

Whoops, I just noticed that we aren't designing Purple as a color, just making certain powerful cards be "Unique Color" and the need to draft certain lands to be able to cast those powerful cards.

On that topic, I think it's a bad idea in general. It adds a whole extra layer of unneeded complexity, will nullify most of what made those cards powerful in the first place, and ultimately get ignored by the drafters for the solid, on color cards that they don't have to splash for.