Unique

Unique is a theoretical static ability that has been discussed by Mark Rosewater, but never printed. If the "legend rule" were eliminated, unique would be added as errata onto a select few legendary cards to prevent them from becoming overpowered.

Description
Unique would give cards the same drawback currently associated with the legendary supertype. If a player controlled two or more permanents of the same name with the unique keyword, that player would be forced to choose one of them and put the rest into their owners' graveyards.

Rationale
Mark Rosewater advocates for eliminating the "legend rule," which he sees as detrimental to the game. According to Rosewater, players of Standard, Modern, and other non-singleton constructed formats are less excited by legendary cards because of their inherent drawback. These players generally avoid playing four copies of a legendary card in their decks for fear of drawing redundant copies. This creates a conflict of interest with Commander players and Vorthoses, who generally want powerful or splashy creatures to be legendary.

Eliminating the "legend rule" has the potential downside of making some legendary cards overpowered. Rosewater suggests adding the unique keyword as errata to these cards, so that in practice the "legend rule" would still apply to them. According to Rosewater, only a small number of legendary cards are powerful enough to warrant the keyword.

Likelihood of seeing print
The majority of R&D disagrees with Mark Rosewater's stance on the "legend rule," so unique is unlikely to be introduced into the game any time soon.