One significant plot point in Return to Ravnica block so far has been the uprising of the downtrodden non-guilded Ravnicans in the so-called Gateless Movement. These proletarian activist have been referenced in the flavor texts of a handful of cards in Return to Ravnica and Gatecrash, such as Fencing Ace (His prowess gives the guildless hope that they can hold out against tyranny.) and Firemane Avenger (Firemane Nevena led the victorious assault on the Barbu Rooftop rebels. Now the Gateless have marked her for death.) There are some indirect references to them coming into conflict with the Boros Legion in particular, such as on Knight Watch, Urbis Protector, and Furious Resistance. So far, though, all we've seen of them in the two fully guild-focused sets that started this block off has been those occasional flavor text mentions. But those offhand references are probably a hint. We can expect the Gateless to show up once again in Dragon's Maze, and perhaps they'll influence the outcome of the race to complete Grand Arbiter Azor I's Implicit Maze.
As for how they'll function mechanically, they're an organization on Ravnica that is parallel to the ten guilds, but also entirely apart from them. They're a loose collective united only by a hatred of guild dominance, and calling them an "eleventh guild" would be anathema to them. It's unlikely they'll get a keyword of their own. However, they might support a kind of mechanical theme to distance them from the guilds.
The one thing the ten guilds all share is that they represent exactly two colors. This means the Gateless could conceivably be about one of two things:
- Monocolor matters. The Gateless might view the confluence of two colors in the guilds as an unnatural conspiracy, one that makes them less than the sum of their parts and inevitably leads their missions to be corrupted. Under this theory, the solution is a kind of egalitarian mutual isolationism, with none of the five colors interfering in the affairs of the others, striving instead for its own perfection. Mechanics used here might be greater amounts of protection from multicolored and similar abilities as well as spells that reward you for spending only a single color of mana on them.
- Omnicolor matters. The other extreme is true unification. The Gateless may believe that Ravnica's problems come from the guilds being set against each other, and that the city-plane would be freed if it were ruled by a single entity comprising the functions of all the guilds. This theme is harder to implement in large numbers - it's difficult to have a lot of five-colored cards logistically - but there might be some abilities along the lines of "add WUBRG" to your mana pool. Additionally, with a new cycle of Nephilim being teased for Dragon's Maze, the four-colored creatures could focus on unlocking the abilities of their "lost" color.