The most popular plane in Magic lore, with some possible close contenders but still the clear leader, is Ravnica. The Planewide City has been the basis of two enormously successful blocks featuring the ten guilds, each of which got a new keyword to call its own in both iterations.
However, this expected structure presents a problem for future versions of Ravnica - there are only so many times you can design ten original keywords and make them different enough from one another. Thus, previous keywords were considered for Return to Ravnica block, a suggestion that was shot down midway through design. The sheer love that the playerbase has for Ravnica means we'll be seeing another block set there, so it's quite possible that returning keywords aren't out of the question for that block.
On this note, when bcrossfire asked Mark Rosewater on his Tumblr blog, "When Ravnica 3 inevitably rolls around, do you think you'll do another list of previous mechanics you'd consider bringing back for the guilds? By then, blocks that are recent now should be eligible for returning mechanic status. How recent before the Ravnica 3 block would you consider looking? If I recall, you listed a couple Scars mechanics for the RTR list (Battle cry for Boros and Proliferate for Simic)," Rosewater said:
How would people feel if we returned to Ravnica and every guild mechanic was a returning mechanic?
Responses to this were overwhelmingly positive - everyone has their favorite keyword that they want in Standard again. User masamunemaniac said "This would be okay. Plenty of design space left with existing mechanics," while zar25 voiced a common sentiment, saying "If you could get 10 mechanics that have interplay between the colors, I think it could be a pretty sweet Time Spiral-y experience. There *would* need to be something new though, imo. Like Fuse cards, or something." A more conditional response came from fatherofmachines, who said "Depends on what mechanics exist by then. Dimir and Izzet still haven’t been handled as well as the other guilds."
How do you feel on the issue of returning guild mechanics? Got any suggestions as to what guild should get what keyword? Let me know in the comments.