One of the most infamous stories in recent Magic design is the one about the Avacyn Restored keyword that could not be. Early in the design of the Innistrad finale, the "good" side - Humans and Angels representing every color but black - had a keyword that did something unprecedented, which Development considered to be so rule-breaking that it had to be replaced with another keyword - miracle - that also represented a completely novel type of design and even got special card frames to denote it.
Recently, Mark Rosewater took to answering a handful of questions about this "forbidden" keyword on his Tumblr blog that might give fans of what could have been hope for the future.
When danvogelsong asked "Was the mechanic called "Forbidden" or was it too descriptive of what it does (like buyback - It says it on the tin!) that you just refer to it as "Forbidden"? Just a poke, I really hope that a Conspiracy set uses an ability named "Redacted". Not a mechanical suggestion, but that'd be a fantastic name," Rosewater confirmed that the mechanic was literally named forbidden.
And when davestrideronahonchkrow asked, "In re: Miracle Avacyn, I thought you said that she started out with a 'forbidden' mechanic that didn't make it into the set, but you couldn't tell us because it could come back in the future. (I believe in you little mechanic. You can do it)" Rosewater replied:
You are correct, the very first Avacyn had the forbidden mechanic, but the one I designed after that went away was the one that made it through the process.
Given that Avacyn's first card design had forbidden, it's reasonable to guess the mechanic was supposed to represent the power of the Helvault that entrapped her, or the unapproachable holiness of Innistrad's Angels.
When openfire1 asked "Is the reason that we can't see what the forbidden mechanic does is that you guys are saving it for an upcoming set? Or is it something else entirely?" the response was:
I think I can salvage it. I’m sure any developer that reads this will disagree.
Innistrad infamously had multiple mechanics that presented fairly harsh Development problems - double-faced cards and the aforementioned miracle were chief among them. Perhaps forbidden was sort of the straw that broke the camel's back, and might be better-received in a less complicated design environment.