"The set to follow Return to Ravnica will be Gatecrash, as announced at the Magic Panel at San Diego Comic Conby Mark Rosewater.
Unlike Ravnica block, Gatecrash will also be a large set — both sets will contain 5 of the original 10 guilds. Return to Ravnica will include: Azorius, Rakdos, Selesnya, Golgari, and Izzet. While Gatecrash will contain: Simic, Dimir, Boros, Orzhov, and Gruul.
Both Return to Ravnica and Gatecrash will be drafted separately as standalone sets — until the release of the third set, “Sinker” (which contains all 10 guilds), when all 3 will be drafted together akin to a standard-sized block.
Also of note: At the prerelease for both sets, each player will choose a guild. One of the six packs in the sealed pool will be a special “guild booster”, and contain a guild-specific prerelease promo card — which, for the first time, you can now play with in the prerelease itself!"