Evergreen keywords, or those that are deemed universal, versatile, or crucial enough to appear in every (or almost every) set, have not changed much since Magic began. Most of the examples are creature keywords like flying, and a select few others are integral to specific kinds of cards, such as enchant or equip. Even slap-'em-on-anything mechanics like scry and cycling are considered at best reusable keywords that have good chances of returning in any given block, not things to always see.
But the only real constant in Magic is change, as Mark Rosewater asserts on his Tumblr blog. When he was asked by immerwulf, "What's the likelihood of an ability we know (and perhaps love) going evergreen? If it's high, are there any contenders?" Rosewater gave an uncharacteristically concrete answer:
I can’t give details so I’ll just throw out this little tease. The evergreen world of the future is not identical to the evergreen world of the present.
While Rosewater may say things that have tricky wording in order to confuse would-be spoiler seekers, he never lies in regard to the future of Magic. This means there definitely will be a keyword newly considered evergreen in the next two years (as that's only as far ahead as R&D works in terms of finished design). The most logical place for an evergreen keyword to debut would be a core set, so perhaps Magic 2015 is going to add it.
As for contenders: The long-rumored blue combat mechanic would necessarily have to be evergreen to be a constant design tool; and protection and regenerate (or landwalk to a lesser extent) might be replaced with cleaner new versions as well, as those keywords are considered in some sense to be relics.